There are some informative threads right here on Expo; Search "Fire Extinguishers"
What I think I've learned:
Common ABC dry chemical stuff from Home Depot contains ammonium phosphate, which will leave a stuck-on melted crust on whatever it hits that's hot, and also corrodes metal, including electrical equipment. However: It's very effective at putting out fires.
There is, if you look, a sodium bicarbonate or potassium bicarbonate dry chemical material that is rated a bit less effective, but doesn't melt into a crust or corrode your electrics. That'd be my choice for cheap and safe.
There is also the liquid chemical material, Halon (the best, but now, alas, only exists in recycled form, a bit more $$, and is very bad for various ozone layers) and the somewhat less-effective replacement for Halon, Halotron.
I've got 2 lb and a 5 lb Halotron units. I like the idea that it leaves no residue.
Be careful if selecting AFFF foam; Some formulations utilize the PFA group of chemicals, which are toxic, non-decomposing big league endocrine disruptors.