MRS4 is the best if you ask me. just what I've used and am comfortable with. try looking around on forums and eBay for a Motek EXO-2 chassis. hard to find because they were discontinued a while ago, but if you wanna get competetive and have the car handle like it should, I reccomend this chassis.
the penguin LM chassis is nice if you can't find the exo-2.. so it's really a win-win situation no matter which way you go.
also, look for an associated micro VCS shock... I believe thats what the penguin chassis uses.