The GT300 series is one of my favourite series from GT2, and at its best when you race it in one of the cars that are actually supposed to be there, i.e. one of the cars that you race against.
It's been a while, but as I remember it, the RX 7 mentioned before and the Cusco Impreza are "easy" cars, the two Silvias (Daishin and Xanavi) and the BP Trueno are "medium" cars, while winning it in the Celica or the MR2 can be quite a challenge.
I also had quite a good time trying to build "correct" cars for this series (i.e. cars with similar weight, hp and drivetrain (no 4WD)). I remember succeeding quite well with a Cerbera, although it felt a little cheaty for some reason...
Too bad there isn't a series like this on GT3. The closest would be GT World Championship on Amateur I think.
GT300 cars on GT3 would be the C-West Razo Silvia JGTC and the MR-S Autobacs Apex JGTC?