I would not recommend to get one unless you have enough cars to finish the game with no difficulty, I haven't done the glitch too, but I made a LOT of grinding, enough to buy 5 of them, but I decided to get a bunch of Le mans prototype and other quite expensive cars instead.
A 20 mln car means winning the Red Bull standard championship 9 times (around 3 hours of play), the fastest way to get credits so far, so choose wisely.
The only one I have is the Ferrari 330 P4 because is my all time favourite car. The other good choice is the 250 GTO also because is new to the series, the Mark IV and the XJ-13 are awesome too, a difficult choice...
Pretty sure you would not need such an expensive cars for the carreer mode so pick the one you like the most.