Interesting to note if you wanted to do the event list while remaining using the same manufacturer to help build affinity, there is only 1 maybe 3 manufacturers that have cars in every class. Chevy is the only one that has every class covered without any tuning (can you tell that GM sponsors Forza?)
Aston Martin & BMW has most of the classes and you can probably work some tuning for the rest of the classes, but, other than that's it.