With regards to decent engine sounds, ive already had a bit of a play using lfstune and with a little shift+a tweaking, ive got some pretty realistic v8 sounds. Using lfstune, on trg, ive found a 5.7L v8 to be one of the best sounding, the bigger the cubic capacity, say 7L, the more ferocious sounding it gets.
Cars i would pay through the nose to see in S3 would be the LX8 (ive seen some photoshopped concepts) and a GT40 type car that could be added to the FZR class, although it would be nice to see another class altogether with, as some have suggested race versions of the Viper or BMW tourers, with a performace span somewhere between the current tourers and the open wheelers