I've been holding onto about 3.5mill credits waiting for an F1 car to show up!
Got the Furai on about day 6 it turned up pretty quick, so I snatched it up. Nurburgring is a scary track in GTPSP with Pro Physics and the cars are difficult to control. The Ferrari Enzo is my current favourite lapping the 'Ring just under 8minutes with no aids and default settings.
Just for reference, any race/drift/time trail is a day in the GT world. If you go to challenges, if you can do a complete block in one GT day. If you quit half way through a block, it will count as a day. So if you see a car come up you want and can't afford, jump into a challenge block and complete it, only one day will pass, as the dealerships change every 2 days giving you a chance to win enough money to purchase the car.
That or you could just do 99laps of Nurburgring for a million dollars