Too late to fire up GT4 now and check, but I'm pretty sure you can just buy it from the M-B dealer (in Germany) in Gran Turismo mode ... assuming you have enough money!
If you're in Arcade mode I think a lot of the cars (and tracks) don't appear to start with.
BTW the full list of M-B cars is as follows:
Mercedes-Benz 190 E 2.5 - 16 Evolution II 1991
Mercedes-Benz 300 SL Coupe 1954
Mercedes-Benz A 160 Avantgarde 1998
Mercedes-Benz AMG Mercedes 190 E 2.5 - 16 Evolution II Touring Car 1992
Mercedes-Benz AMG Mercedes CLK-GTR Race Car 1998
Mercedes-Benz Benz Patent Motor Wagen 1886
Mercedes-Benz CL 600 2000
Mercedes-Benz CLK 55 AMG 2000
Mercedes-Benz CLK Touring Car 2000
Mercedes-Benz Daimler Motor Carriage 1886
Mercedes-Benz E 55 AMG 2002
Mercedes-Benz Sauber Mercedes C 9 Race Car 1989
Mercedes-Benz SL 500 (R129) 1998
Mercedes-Benz SL 500 (R230) 2002
Mercedes-Benz SL 55 AMG (R230) 2002
Mercedes-Benz SL 600 (R129) 1998
Mercedes-Benz SL 600 (R230) 2004
Mercedes-Benz SL 65 AMG (R230) 2004
Mercedes-Benz SLK 230 Kompressor 1998
Mercedes-Benz SLR McLaren 2003
Some of those are 'prize cars' though, awarded for winning particular races or championships.