I used a Dodge Viper Team Oreca Race Car for my B-Spec driver, fully tuned of course. I don't believe he won it on the first try, but it worked eventually. This was 2m at the dealer.
I just bought the GT LM Spec II Test Car to see if it's any better at spamming the Indy track in that series for A-Spec, and it seems to be slightly faster than my Viper and GT LM Race Car Spec II (although it weights a little more while having similar hp). I don't know how it will do outside of Indy yet though or how well the B-Spec driver can handle it.