I would definitely have a coccidiosis med on hand, like Amprolium or perhaps a sulfa drug. When you find sign of coccidiosis, the clock is already ticking as intestinal damage has already occurred. There are several newer folks on the forum who have found this out the hard way ~ and the feed store isn't open at 11 at night.
I personally keep 3-4 different categories of worming meds on hand, most of which I have not yet used.
I keep Terramycin on hand, but I should probably also order some Tylan and/or Baytril.
First aid supplies are good too, like Blu-Kote, iodine, neosporin, and I think the prev poster mentioned saline solution. I keep a collection of syringes, a couple of needles, and God forbid I should ever have to use one of my sterile, sealed scalpels.
I use vaseline and Prozap poultry dust (pyrethrin) on a regular basis, so I have those on hand. I also use food-grade DE.
I use Oxine (chlorine dioxide) for cleaning and for application in the coops and water, but this is my own personal preference.
I keep a supply of probiotics (Immuno Charge from McMurray), vitamins/minerals (Avia Charge 2000 from McMurray), and electrolytes that I rotate through the water supply, but these are not emergency items.
Probably the most important thing I have is a collection of vet information on diseases/disorders, and a dosage chart I have been working on. I keep this in the vet med cabinet, and I also wrote my dosages on the med bottles with a fine sharpie so if I am in a pinch I don't have to run around looking things up. There are others on the forum who have shared med charts in the past, and when mine is in decent shape I will do the same.
It's wise of you to plan ahead! Good luck.