Lucky if you have these already, not cheap to buy with in-game credits. Will be useful for new players at Christmas. And you can redeem this now if you activate GPU early. Yes you can get it later too. purexbox.com/news/2020/10/forza_horizon_4_perk_now_available_with_xbox_game_pass_ultimate
According to purexbox.com:
"you'll be able to get behind the wheel of the 2005 Honda NSX-R, 2011 Koenigsegg Agera, 2010 Pagani Zonda R, and the 1938 Maserati 8CTF, all at no extra charge!
This Perk is valid for the Xbox One and Windows 10 PC versions of the game, as well as the upcoming Xbox Series X and Xbox Series S versions, and is available to claim until January 14th, 2021.
To claim it, simply head to the Xbox Game Pass section on Xbox One or the 'Perks' section on the Xbox app for Windows 10, and don't forget that plenty of older Perks are still available to claim with your membership as well."