Also if you go into Forza hub (an app on xboxone) and select "last reward tier"
It will tell you what stuff you have accumulated already for when you get Forza 6.
So I've just hopped in and it says as I stand now I'll get 7,100,000 cr and 11 cars.
So I know from that, that I'll get all the cars except the Audi R18 which need tier 8, but I'll get that when I get there and I need more cars in my garage to boost that 7,100,000 to 25,000,000.
My wife says she'll bring me Forza 3,4 and FH home one evening so I can fill up my profile with the 300 cars I need, result.
Worth checking to see what you need, might be an insentive to play some more