If you want to buy a racing game, I'd say don't bother with LFS at this time.
There isn't a lot of racing going on, and updates to the game are very rare.
If you do decide to get the full game, I would definitely advise against purchasing an S3 licence. S3 has been out for years and the only extra piece of content (compared to S2) is one track. And it's £12.
You're better off going to the forums (if they have them) of the servers you were banned from and ask for an unban request. Or you could ask the owner or person who banned you, depending on how they operate as a server.