I'm as happy as the next guy to see Spintires revived, but for me everything hinges on how you intend to maintain this game.
I think the game is already good, and to be honest I don't think the Mudrunner release features are enough to warrant the price of an entire game. Challenges and new map is neat enough, but that's the sort of thing I would expect in a patch, or possibly a small DLC. Anyway, I understand that you are a new publisher that needs to make money from this revival, but from a customer point of view it makes less sense.
What I'm interested in is your plans for the future of the game. Features and content that you are planning to implement or are actively working on. Things that would make a Mudrunner upgrade make more sense. Is this something you're going to be talking about? Perhaps prior to release?
The game has a lot of potential, and judging by your track record as a publisher I'm hopeful, but so far challenges seem like the only significant difference, which - while neat - is not really enough. I'm assuming there is more to come.