Iam sharing it with a landscaper 50/50 (big mistake).
Is your partner in this venture a professional landscaper?
If so, he'll very likely be putting more hours then you will (assuming you are using strictly as a home owner).
Since you're already sorry you got into this deal, can you get out of it?
Do you have anything in writing that solidifies the deal?
If not, you might want to rethink the situation.
Also, are you at all liable if he has an accident at a jobsite. Will he be allowing any others (his work gang) to operate the machine? Should his business go under, is the tractor on his books as an asset (then it could be seized in a bankruptcy)?
I may be being too cautious or paranoid (watch the X-Files too much!), but I would want those answers before I ventured too far in the endeavor.
All that said, I'd base any repair/maintenance costs on hours used by each partner...a ratio, perhaps (i.e. he puts 10 hours on, you put 5...he'd pay 2/3rds the cost of repairs. Any insurance should be paid by him...your insurance would be covered under your homeowners policy, most likely. However, if the machine is used for commercial work, your homeowner's policy may not...probably won't...cover any damage whatsoever.
This might give you an out, if you want out.
And, lastly...can you back out of this situation if it becomes apparent it's not working? I hope he's not a relative!
Now, my machine is a 1991. It had 600 hours on it when I bought it in 2001...works out to 60 hours per year.