Well, there is this tuning shop which offers the same thing in pieces. And that's Gran Turismo for you. It gives every one of us the opportunity to become a racing bully if we want. I've done it a few times myself to the bots, when all I really needed to do is turn down the difficulty... silly me.
The UCD does need an overhaul though. Often, there are only 14 cars available because one is sold out, taking up a spot where a new gem could be found. And that greyed out block can sit there for weeks.
Here in the States, there are massive used car dealerships offering dozens to well over a hundred cars on any given day. I hope one of these updates to give us something more like we had in GT4.