Well it depends on your situation, if you've got plenty of many, then buying stock and modding wouldn't be a bad idea. But if you're working on a some what limited budget, i would probably looking the best car I could with the most mods, if the car's been kept tidy, chances are it's been looked after somewhat by an enthusiast. If the car looks trashed, it's probably been thrashed.
But if you're buying a modded car, try and budget that little bit extra for the un known. And the other thing with buying a stock car, just because it looks stock, you could also run the risk of it not being looked after/serviced regularly.
That's just my opinion, but I think going through Geoff you'll get a pretty good idea of what the car is and has and needs.