I take it from your other post that you managed to get the CTR...
Anyway, the best way to earn cars for Time Attack and Free Race, all of the cars, is to earn an S for every stage of Driving Revolution. Otherwise, just use German sportscars for the RUFs, and GT racecars for the M3 GTR (in Enthusia Life you will need to find a race that has an R-class car as the prize, and hope it's the M3).