Duke best GTA 4 car <3
Trading wins is definitely the way to go. Unlocks used to be even worse (50 Rally wins to unlock brushed steel, wtf). Once again, "game design" with 0 understanding of player behavior. They should have set a number of race participations (or even reaching the end), but noooo.
As for slavmobiles, I personally recommend the Regina, Futo, Warrener, Primo, Glendale and Ingot.