That McMaster site is really something! But then again, its an online store - can't visit them physically to get assistance.

Imho stainless has not not benefit for RC cars; they're still soft enough to strip their threads.

I buy "high tensile" from Coventry for use on motor screws (both gas and electric) and for suspension components. Chassis screws etc don't really need HT, but sometimes HT is the only one stocked on the shelf.

Haven't stripped a HT thread or head yet... but some of my motor cans are starting to look deformed from overtightening!

Pricewise I think approx/under A$1 each for M3s in single quantities. (Still cheaper than buying spare screws from HPI etc!)