I see your point about voiding warrantees, but purchasing a used Evo is a very risky proposition, because almost all of them have been modded and/or abused, then put back to stock and sold to the dealer (or another person) with a slipping clutch, busted t-case, and worn brakes.
If you can find one that has never been modded, then go for it, but still be sure to check everything closely.
If you buy an evo you have to do 1 of 2 things. Either mod it or don't. It's not worth the worry about warrantees if you want to mod the hell out of it. If you decide on getting it Fred Beans of Doyelstown set me up right.