Forza Motorsport 5 Forza 5 buying guide? Do i need one?

Ok, so I've been really into forza horizon 2. picked up a tx and a playseat challenge. loving it.

so anyway, right now forza 5 is on sale for 30 bucks in the xbox store, so im thinking I might pull the trigger. but im reading a LOT of stuff that I apparently missed out on with regards to its micro transactions.

with horizon 2, I just got the ultimate edition, I know im covered. but they don't have that for forza 5, so my question is......

what are must buy dlc's, what cars come with the game, which ones are in each of the packs etc... google foo is failing me. anyone have some links or advice?