Essentially, Forza is the best arcade racing series available on PC. At the time of the game’s release, I saw a couple of reviews and met people broadcasting the opinion - “There is nothing new in the 5th part!” And you know what... For a person who does not understand the issue, this is really true. If you just want to drive a few hours of racing, just try, and are not a fan of the previous parts, then it really makes sense for you to buy the 3rd or 4th part at a discount and not worry. You will have practically nothing to lose, and the game will give you a lot of fun.
If you want to get a game on which you will spend 500+ hours and which will encourage you to log into it periodically, then of course you need to buy the current part with all the additions. I won’t explain all the advantages of a live game with current updates and regular release of new content; you should understand this yourself.