Depends on wether you are going to race on public servers or orgainsed events. check out the rfactor racing club, and if you see and upcoming event you like, check out the thread as it has links to all the downloaded mods you'll need to race that event. for instance, ive only downloaded the mods i want to race in, ie V8's, and some of the formula cars. its up to you.
if you want public servers (which can be dodgy) boot up rfactor and check out what are the popular mods that you would like to drive in (wether it be stock cars or formula racing, rfactor really has everything) and then search for them online. i would only download the ones you need, the more you get the more chance you have of having trouble with one of them.
