How do you decide what to sink your money in? Well, stick to the cheap stuff and don't hold what you can get fast.

Medicine Cabinet - What's On Hand?
Medicine Cabinet Recommendations - by Doc Johnson
Protozoan Disasters:

SALT: Always have salt on hand.
Astor, Akso Nobel, or Morton brand non-iodized salts sufficient to treat one system to 0.3% two times. Most any salt is okay if it's not: YPS laced, mineralized, iodized, or really dirty. I've used animal feed or licks - "salt blocks" from the feed store with excellent results, you need to read labels and look for 99.5% purity with no trace minerals. The blocks are easy to store and transport. Put them in the way of the water return and they dissolve rapidly. NEVER put salt into the filter intake. The sudden rise in salinity will annihilate your filter bacterial function. One man local to me put fifty pounds into his skimmer, straight through the filter, and made his beads snow white. (Could that ever be a plus?)

Potassium permanganate
Here's the tricky part. When you're using Potassium for protozoans, you should use it daily. This is because the reproductive cycle of the protozoans is faster and they can amply repopulate between treatments.
Potassium permanganate (Using Potassium For Trematodes and Bacterial Infections)
This treatment strategy can spread out the treatments and it requires peroxide to be applied between treatments. You run your filter between treatments and make sure the pond is as clean as possible when you start. The four day regimen accounts for the Flukes three to four day life cycle and is not as aggressive as the above therapy.

Formalin
I personally don't keep Formalin on hand. It "goes bad" in about a year's time if improperly stored, and is very sensitive to light. Since it's readily available locally, you can land some most any weekday at the pharmacy or with prescription. Sometimes, the Formalin is stabilized with Methanol which helps make it last longer but sooner or later you'll see white precipitate in the solution. This is para-formaldehyde and is DEADLY toxic to fish. I once tried to put my Formalin through a coffee filter to remove the para-formaldehyde and still killed all small fish with the otherwise clear chemical. SeaChem HealthGuard is not actually Formalin but a close relative which puts less of a beating on fish.

Crustacean Disasters
These are the parasites Anchor Worm and Lernea. They are fairly common in the latter part of the summer. Beats me why. They do not require that the fish be under stress to get a foot hold. They spread like crazy. They can come in on plants, water, ornaments and unseen on recently infected fish.
Dimilin
Dimilin is cheaper to use in larger systems. It's like, $5 per gram and that treats a thousand gallons. Usually one treatment suffices unless you have high water turnover.
Program
Program is from your vet. You can simply buy it for your large breed dog. It's explained above. Problem with Program is then it costs like $8-10 per thousand gallons to treat which is essentially twice what Dimilin should cost. Still, sometimes it's faster and easier to get, and time matters.

Water Quality Disasters
These include over-doing it on a water change, which has claimed the lives of one of my collections, and I am aware of numerous other glorious collections of fish which have been killed by accidentally forgetting the water change and leaving chlorinated water running all day or night.
Ammonia Spikes (Ammo Lock II)
Ammo Lock II is the only Ammonia binder which does NOT use a modified aldehyde or an unstable sulfide ion binder. It turns out that the sulfide ion binders are extremelty unstable. The sulfide ion binders originate in China, Japan and elsewhere and are formulated on a Sodium Formaldehyde Sulfoxylate ion called "Rongalite" which usually lasts about three days in solution.
Other compounds use massive amounts of sodium thiosulfate, which does nto bind ammonia at all. Sodium thiosulfate will defeat the ability of your ammonia test to detect Ammonia.
Still others use modified aldehydes which are arguably as toxic as the Ammonia itself.

Dechlorinator
Yeah. People leave the water on. They also add accidental stuff to their water and have to do large water changes at a moments notice. Usually it's Sunday when the store is closed, or they need thousands of gallons worth all at once. Dechlor is important. I tend to keep three times the anticipated need on hand.

Nitrite Spikes
You don't have to use much salt to curb Nitrite absorption. One pound per one-hundred gallons is enough. The length of protection afforded by salt varies from species to species, but Koi and Goldfish can be protected from Nitrite uptake for several weeks. The protective effect of salt on Nitrite uptake is by interference with sodium ion channels in the gill membranes and is NOT permanent.

Potassium Permanganate Overdosage
Get 3% USP Hydrogen peroxide at the local drugstore. If you ask the pharmacist, he can order you a whole gallon at a time and save a few cents. Hydrogen Peroxide will deactivate and de-colorize potassium permanganate treatments. The dose is 1 pint per five thousand gallons which can safely be increased to a gallon per five thousand gallons for more rapid and complete clearing. I tend to stay on the low side and wait longer.

Low Dissolved Oxygen
Get 3% USP Hydrogen peroxide at the local drugstore. If you ask the pharmacist, he can order you a whole gallon at a time and save a few cents. Hydrogen Peroxide will increase dissolved oxygen when applied at one measuring "cup" per hundred gallons. Do not exceed this dose. You may also use a squirt bottle, and apply one long "squirt" per gallon of water. Last time I checked, "a squirt" is between 0.50 and 1.0 milliliters.

Test Kits, What's Important and What's not.
Some test kits are really important. Go to my home's pondside and you will find weathered, raggedy-looking boxes of test kits. I am fanatical about pH and Total Alkalinity. You absolutely must have tests for Ammonia. Nitrite. pH and Nitrate. There's more to read about these parameters when you click here.
I always have the following, ranked from most important to least important:
1) ph
2) Total Alkalinity Dip Strips
3) NitrAte
4) Ammonia,
5) Nitrite

In General
I don't have, and rarely care to check; Dissolved Oxygen because I am relatively fanatical about flooming and other forms of aeration. (More detail) (Another link)
I also don't think a person needs a Hydrogen Sulfide test except when I was testing my Aquascape pond.
I also don't have or care about General Hardness. The mineral content of my water is not a big deal for me. It's the KH (Carbonate or Total Alkalinity) which specifies the "distance" separating you and "pH crash"..

Equipment Handling Equipment Wisely Chosen
A Nycon net of 24" is ideal. There are larger and smaller varieties but you can do what you need with the 24" net. Never carry a fish in the net. Always transfer to a Koi Sock or a blue bowl. Nycon nets can be equipped with extendable handles. These suck at long extension because the handle becomes so flimsy that the net lags behind the fish. You need a long, stiff handle to be able to maneuver the net under the fish.
Plastic bags are also nice to have. Large bags can be used to wrap the fish in for injections, or other treatments. When you have large plastic bags, you can bag a fish to carry to the vet or to a show. Bag the fish under pure oxygen, which is available at a lot of pharmacies for older people with emphysema. They rent the tanks for eighteen dollars per month. A regulator may be extra.

Drugs to Keep on Hand, Ones to Avoid
Chloramphenicol - Not usually had on hand. It requires reconstitution and is not stable for long after reconstitution. A safe medicine.
Baytril (Use)- I have this on hand for my fish. It doesn't need refrigeration, it's got a long shelf life and it doesn't require reconstitution. Approved for use in animals.
Amikacin (Use) - I usually DO have this around for all the same reasons as Baytril: It doesn't need refrigeration, it's got a long shelf life and it doesn't require reconstitution. Approved for use in animals. It's just NOT as safe for repeated injection and in smaller fish.
Azactam (Use) - I used to have this around, but mostly for my customers in need. It's like Chloramphenicol in that It requires reconstitution and is not stable for long after reconstitution unless you freeze it promptly. A safe medicine. Not approved for use in animals.
Other Drugs - Are listed in the book. There are about 110 different medications discussed in the book. For a complete list: Click here.

Feeding Koi and Foods.
I feed a mixture . I like to take a super affordable staple, mix it with a dark brown oily pellet, then scatter some Krill in it. If you ever get a crack at Silk Worm pupae go for it. Just, not too many. Also, grapefruit and melon slices.

Stuff I *Don't* Generally Have Around
I don't use phosphate binders. No abrasive gamefishing, or pool nets. I use Chloramine T, quite a bit, but I don't invest to have it on hand for my own fish. I don't have Koi Vitamins to add to the water. I don't vaccinate the fish. I don't have NuFlor, Cefotetane, or Rocephin or the other odd antibiotics, prefering to buy them on the odd occasions I need them. I don't keep copper sulfate on hand because Oodinium is exceedingly rare. I don't keep Formalin around because of its short shelf life. I keep Oil of Cloves handy for minor procedures but the IsoFlurane is under lock and key.