The things I tend to use the most are:
Triple antibiotic ointment (aka neosporin)
Gauze/cotton balls for cleaning wounds
Thrush product such as generic coppertox
Scissors (for vet wrap and general use)
There are all sorts of fancy wound products out there, but generally I haven't met a wound that triple antibiotic couldn't handle well. In general, that is my go-to wound treatment.
And thermometer/ stethoscope would be great. I still don't own a stethoscope but it would be much better than sticking your ear to the horse's belly like I normally do (when listening for gut sounds).