Well, the R390 is probably the best car of the four. The race version, at least is one of the best cars in the game, I think. If you don't know, there are three real life Le Mans cars in the game--the R390, GT-one and 787B--you'll want at least one of them.
I think the road car is close, except for the decals (of course) and various other race upgrades, like the very important full racing transmission. You can add that, though. If this was the race car, I'd say it's a no-brainer and go for the 390. But, if you plan on getting the race car later (and I would), you might want to look at the others. I'd consider the GT40 just for both nostalgia, and my thinking it's a pretty cool car.
I haven't spent too much time with the Jaguar or Opel, so someone else would probably be better suited to comment on those two.