Had a look through past UCDs. If this game is similar to GT4, I can see an 80s, 90s, 00s UCD cycle. Throw in the 70s and earlier cars as rare to find. To me, keep the prices low enough to obtain and it'd be better for casual players to buy when they pop up.
(already known and probably mentioned a few times)GT Sport, initially, seemed to cut cars off at 2009->2017. I think the same could apply. 2010->current in the NCD.
When it comes to race cars, I'm thinking those should be separate from the UCD.
GT2 UCD organised by dealer
PD could still change it up, by rotating cars available.