No cars awarded for completing Arcade Mode races appear in your Sim Mode garage - they are added to the cars you can select in Arcade Mode. They are:
Area A - Mazda MX-5 89 (C-class), C-West Silvia (S-class)
Area B - Audi TT 1.8T 225 (C-class), Lancia Delta HF Integrale (R-class)
Area C - Mazda RX-8 (A-class), Gillet Vertigo (S-class)
Area D - Nissan Z Concept (A-class), Pagani Zonda Racer (S-class)
Area E - Ruf RGT (A-class), TVR Tuscan (A-class), Panoz Esperante GTR-1 (S-class), R390 GT1 LM (S-class)
Area F - Toyota GT-ONE TS020 (S-class), Mazda 787B (S-class), Viper GTS-R 02 Concept (S-class), TVR Speed 12 (S-class)
Half (the first one for A-D, the first two for E-F) are awarded for completing on Normal. The other half are awarded for completing on Hard.
However, if you complete Arcade Mode Time Attack/Time Trial, you will be given a Panoz Esperante GTR-1 in your Sim Mode garage.