I used the VGT Mercedes at around 550pp. I also had good luck with the TVR Speed 12 at less than 550pp. It has adjustable downforce which helps with the rainy events. I added nitrous for the long straights at Sarthe and it won pretty easily. As far as setup goes, keep the suspension soft and the ride height relatively high to aid grip. Put TCS at 1 if you're getting too much wheelspin out of the corners.
One of the tricks used to be to use RH tires, not wet, because strangely they were much better in the rain. I don't know if that is still true, or if it was "fixed" in one of the updates.