I'm currently doing the 4hour Roadster endurance race and am thoroughly enjoying it. I bought the oldest premium available from the dealership (I think it's an '81) and performed just weight reduction, mostly to help tire wear. The car is more than 50hp down on its rivals but about 200kg lighter.
Running on Sports Mediums is making it an interesting race. When on new tires, I'm about 1-2seconds slower than the leader, but due to how close in performance the cars are, the AI has trouble lapping other drivers, allowing me to catch up. They lap the opposition about once an hour. I'm just on 2 hours in and have just had my 2nd pit stop where the leading AI (ahead by about 7 seconds at the moment) hasn't pitted yet.
Maybe to make it more interesting would be to run racing hards, and probably not do any weight reduction mods at all.