This is a fun event. The last race is the toughest, I think. Eifel is under-used in GT5 but I don't know why. The difference is the hills, much like the tarmac track. You can't see the road ahead but that's the fun of it.
A buddy on here lent me his Alfa 2.5, the screamer, and I won 3 of the 5. Yes it's a race car but it's underpowered. Didn't have the juice for the last one, however. I used my Lambo Aventador for Fuji and it was perfect for it, and the 2J for Eifel. Lame, I know, but in all of these it was such a thrill to pass the Miuras after sucking exhaust in the Lambo event.
Tip: If you haven't tried the Alfa 2.5 '93, give it a shot. It's rock stable and oh so fast. Plus it revs around 11,500. Keep the revs up and it's a rocket. Sweet exhaust sound.