I've done a lot of credit farming, mostly in b-spec. I currently have ~5.8m. I find that doing the second race (forget the name of the track) in 'Like the Wind" with the Toyota Minolta Race car (the one you win from this event) is a good and quick way to save up credits. You only get 70k, but with the Minolta race car, it only takes me ~3:30 to win the 5 lap race.
Can also do the Daytona Oval in the American Cup tournie (Expert i think). Gives you 98k and about 4000xp. I've been using the Nascar vehicle. vey straight forward. Could amass a million in about an hour or so. I'm guessing the Formula GT will appear in the used section? How much are we looking at?