You need to complete about 36% of the game on gran turismo mode to unlock all the tracks for arcade mode. La Sarthe is one of the last to unlock. To get the Bentley you need to win the 2nd Le Mans 24 hour race i think.
Get yourself the Toyota Minolta from the El Capitan endurance race and let your B-spec driver have a go. He might win it, or he might just keep stacking it going down the Mulsannne, they need practice to make them any good (a bit like you).
A handy hint is to win the legend of the silver arrow race in the mercedes hall, and then take the Merecedes touring car you win, and enter the DTM, not only is the prize money high but you win a CLK GTR which you can sell for three quarter of a million. Get your b-spec driver to keep repeating it until you have enough money to buy something silly. A tuned CLK GTR is good enough to win all the non LMP endurance races comfortably and then once you have the Toyota Minolta you should be unstoppable.
Any questions just ask, at last check i was at 90 odd percent complete, with over 680 cars in my garage, you gotta collect them all.
J.