There are 12 missions, the car requirement for the final one is 860, so don't waste money on buying or upgrading cars that can't be upgraded to 860.
Can be done with
- Alfa Romeo 8c Competizione (max. 953) costs 360K
- Ford Shelby GT500 (max. 907) - boss car, you get it for beating turf 2 boss for the second time (attempting to do this will cost you 25 gold!)
- Hummer HX (max. 920) - event car, not sure if you can obtain that
- Bentley Continental (max. 1002) - costs 175 gold, so not for you probably
If you got as far as Turf 3, winning the 12 missions should not be a problem. Especially since you're not going after stars here.