Keep your cars. The last season I did, I sold a bunch of cars I thought I wouldn't need anymore, including the one time cars. After getting 100%, I started over because it just felt so empty and I wanted to take advantage of the 1000 car garage. Started the season over, got 100% and other junk w/all but two cars missing (don't even know which ones, but according to some list, 2 (not the autounion and sileighty either)). It just feels more comlete if you keep the cars you win. All of them, unless they're duplicates of a car you don't really like.