I don't see what the big complication is.
Its simple, 3 tier level the races like the license tests.
The less mods, the more credits and XP for each race.
You want more, move up to the next difficulty level.
Everybody's happy. Well thats probably a little too optimistic.
This is the best way to do it. Kinda like the way the a-spec points worked in GT4. The less advantage the more the points.