I used a toyota GT-One that I bought for 4.5million credits. If you have over 25% completion, there are quite a few good cars that can be attained from endurance races.
Roadster 4 Hours Endurance : MAZDA RX-7 LM Race Car '01
Tokyo R246 300km : MITSUBISHI FTO Super Touring Car '97
Super Speedway 150 Miles : HONDA NSX-R Prototype LM Race Car '02
Nurburgring 24 Hours Endurance : POLYPHONY Formula Gran Turismo '04
Suzuka 1000km : LISTER Storm V12 Race Car '99
Motegi 8 Hours Endurance : HONDA NSX-R LM Edition Road Car '02
Tsukuba 9 Hours Endurance : NISSAN SKYLINE GT-R Race Car (CALSONIC) '93
Sarthe Circuit 24 Hours I : AUDI R8 '01
Sarthe Circuit 24 Hours II : BENTLEY Speed 8 Race Car '03
Fuji 1000km : NISSAN R92CP Race Car '92
Infineon World Sport Car Trophy : FORD GT LM Race Car '02
El Captain 200 Miles : TOYOYTA 88C-V Race Car (MINOLTA) '89