I got Forza 7 just this week. Remembered the BAC Mono was good fun to drive in Forza 6, so I bought one and paid to upgrade it, then went to an S class lobby to use it. Was puzzled why I couldn’t find it in the car list.
The BAC Mono is banned because it is open wheel. All open wheel cars are banned in public online play unless the lobby involves a division of open wheel cars like the birth of Grand Prix or so on. It has nothing to do with it being faster than the rest it has everything to do with open wheeled cars ability to cause extreme havoc on a track if they get into an accident with the current collision physics.
But to answer the OPs question, check the leaderboard. If you want a car with good top end, acceleration, and good handling you will be looking for the leaderboard cars, especially in higher classes like A and S.