As far as it looks there's really only going to be about 2 or 3 cars that'll be faster. The rest of the field should end up atleast fairly close together, most are in a weight range of about 1000-1200kg with varying levels of downforce available to them. The Viper ACR, Corvette Z06 or ZR1, Lambo Murcie SV, Audi R8, Ford GT, etc all are around 1200-1260kg so will be right in the ballpark with each other, cars with less downforce will be a tad further behind those and the GT2 cars with downforce may end up mixing it up with some GT1 cars (just like the real BPR!).
I'd stick with the Viper TBH, 6 races is a short season compared to most of the other online series so once it's over revisions will be easy to make as they'll be rather fresh in the minds of everyone. But, if you're genuinely keen on running with the guys closer to the front of the class maybe go with GT2?