You have the default career mode already in Assetto Corsa. It's not that useful because for 95% of the time, you start from last position and have at the beginning 3-6 laps to finish at least third, so it's ramming time in order to gain all the required positions. If you set the AI difficulty at 98% by default, you can always dream for finishing well, or set it at minimum which is 80% but there it becomes too easy.
If you have the AI on any given number (95%, 100%) is it harder in any way in career mode than if you just do a custom race on the same difficulty?
Don't believe so.