I have a full grid of Group C cars. My favorite race for them is Monza, with Spa next.The C9, 962C and R92CP are good alternatives. They all have the right shapes, but it gets expensive if you want to do half a grid of DPs.
They can be used to fill a grid, rather than drive them. Such as tuned for LMP2 & LMP3 duties.Since I really hate the way the hybrids' battery life is managed in GT7, I never even thought of using them in Custom races.