I found a place that has what I think good prices on all sorts of machine screws. My question is what are the most common sizes used and should I just get all stainless?

3mm ? hex type ?

i tried stainless on my buggy : this steel is just too soft but it's ok in aeras where you don't disassemble too often

So they are all pretty much 3mm? They are listed like this M3x.5x10(of course multiple lengths) The have button head, cheese head, flat head,pan head, wafer head and socket cap..

M3 .5x10
M3 = the diameter
.5 = the thread pitch
10mm = the Length.

Your transmission uses M4 to screw it to the chassis.
Stainless steel looks good, but can be a pain when they strip.
I would recommend 12.9 Black oxide.
Like Tony's screws http://www.tonysscrews.com/product.s...&categoryId=57
socket cap, and flat heads, are the most used