JimR said it best,whatever you have is the best car to autocross.FWD is easier to get started with safely,and there are probably a lot more inexpensive options for simple bolt ons.By safely I mean understeer is easier to deal with than oversteer.You will eventually do both no matter what you drive.
I've autocrossed a not well prepped b13,and a very well prepped b14;so I'm biased towards the b14.I've competed against every type of car(FWD,RWD,AWD,MR,),big horsepower,no horsepower,whatever.I've sent every type of car home disappointed on Sundays.If you put in the time to pay attention to details,listen to those who know,you can be fast in anything.So it boils down to hard work and persistence. Pick a car you like,and are comfortable with,and go for it.