Anything that takes your fancy, that's what I do (now). Before I was on the lookout for any and all Japanese cars, rally cars and so on, but recently realised that I'll be playing GT5 for long enough to see any car I might want in the UCD and likely have money to buy them, so there's no rush. That said, I've only seen the Bentley Speed 8 and the Escudo once each, and I've been playing almost constantly since November.