I like the UCD as it is. In fact, I would actually prefer it if it was even tougher to get at the cars. Cars like the '73 BMW 2002 Turbo and other classics should pop up every six months or so. The waiting and anticipation is what gives them worth. Yes it is a game and they are virtual, but does PD prefer the legions of "nothing else to do in GT5" types?
And the NCD ...I think it is WAY too easy there too! The pricing should be such that it takes weeks or months for a person to be able to buy a brand new Premium car ...and by Premium I mean premium models like Mercedes, Ferrari etc. If, in real life, I was told that I can buy a brand new 458 Italia simply by "grinding" for an hour or so, what actual worth would that car have for me? The harder it is to get ...or the more expensive it is, the greater the perceived worth. In order to please the people who want to get at everything the second they launch the game for the first time, I wish they would introduce a test drive system (a system which allows you to take the car to any track in Practise mode only).
@OP - Take your time ...you will find them all eventually. As frustrating as it may seem, I imagine it would be a lot worse to have the game for 3 months and become part of the "nothing else to do in GT5" crowd.