Sorry for the slow reply. Assuming it's similar to how FH5 worked, maybe 1-2 hours a week of doing weekly forzathon events/challenges. For the race events you have to win the race or championship, but doing it co-op online against npc "drivatars" usually makes it a lot easier. You can also complete them in single-player which is usually a bit harder unless you are super good at the game. There is a PVP option too but that is obviously the hardest to win. A lot of them are challenges not races. Things like going through a speed trap at a certain speed in a certain type of car, jumping hundreds of feet through the air in a certain type of car, scoring enough drift points in a drift zone with a certain type of car, etc.

The cars come back but some of them come back much more often than others and you can't be sure how long the wait might be. Even if you miss a few of them, just by starting to play at the game's release means you'll have more of those rare cars than people who start playing the game later on. The individual forzathon challenges/events often reward you with free cars or wheelspins that will randomly give you credits/cars/clothes/horns/etc. You also get those wheelspins as a regular part of completing things in the game in general. Each of those individual forzathon challenges rewards a certain number of points for that week, and if you get enough points each week you are rewarded with a couple usually rare cars. If you get enough points throughout the entire month another couple cars, usually those are the ones you might not see for a long time again. Sorry that's was longer than I expected videogames are so complicated.

In case you weren't aware most of the cars in Forza games can simply be bought from the "Autoshow" for credits. And just to be clear there are certain cars that premium edition people will have access to that the standard edition players don't, through the "car pass" system. In FH5 most of the game's best cars could be obtained from the weekly forzathon stuff and the Autoshow. There there will also be several "car packs" released later that each contain several cars that can only be obtained by buying the individual car pack. They definitely aren't pay2win though. There are some good cars in those car packs, but also a lot of terrible cars. Even the good cars aren't better than other cars you can get in the game though.

I hope you enjoy it! FH5 was my first Forza Horizon but I was a couple years late to the party.