I would imagine that 90% of the cars in the game are capable of less than 900bhp yet more than capable of winning at the 'Ring ... just depends on the opposition!
Really depends if there are any restrictions.
The best car under 900 is the FGT.
If we're talking about race cars, the NOMAD Diablo can be tuned to 850ish HP and can do under 6:30 lap times. Also, a lot of the LMP cars are beastly along with most of the Le Mans GT cars.
The Zonda R with a turbo pushes 850 and can be very fast.