The official free stuff looks complete rubbish. I don't bother with any of it. It is appalling that they exclusively use it to introduce new people to the online system.
The BTCC cars are, on the other hand, amazing! I pretty much only use rF2 for BTCC and 992 Cup now. They both have incredible physics/ffb and look fantastic. The 4 tracks are also very high quality. (the other tracks are easily found in the workshop).
The only caveats are, 1) the two rwd cars run rings around the majority fwd cars because the tire wear is so different -
it urgently needs a BOP for this, or just accept that RWD are essentially a different class, or just don't use the RWD cars. 2) there is no way to edit the grid to create a BTCC race weekend in single player - except to create a server, run the races on the server as a multiplayer race + AI, use rF2 Log analyzer to edit the grid, and then use an admin command to load the edited grid - hugely frustrating, time consuming, and ridiculous.
Other than those two things, I heartily recommend the BTCC cars (and the 992).
If you don't have the 992 - get it, just get it. I think anyone with a decent ffb wheel should own this car. I don't even like Porsches, but this car is so much fun to drive. The 992 also comes with a PCCGB livery pack, so you simulate a BTCC support series as well.
Oh and the Mini is great fun!