you can buy heavy duty wheels for a good low price at bikepartsusa.com, bikeworldusa.com, and niagara cycle works. look for the ones with 12g (105) spokes, shimano hubs, and weinmann rims.
if you can't afford discs right now, just buy some good ol' calipers. they're pretty cheap.
but you know you're gonna have to buy new wheels again if you want disc, right?
me personally, i love coaster brakes. not the ones that come on huffy's, though. them ones are lousy...