Impressive you can find a parking spot for that price - where are you based?
Talk to the maintenance organisation that you are hoping to get all your maintenance done at. You should get them to do the logbook audit as well as the pre-purchase inspection and get them to setup a program with estimated costs.
There are many ways to do this, but I would be inclined to have all the yearly costs such as annual, 6month check, parking, part of the engine fund, insurance...etc... As a total divided by all the members, this way you are sure that even if she doesn't fly a single hour, there is enough money in the bank to keep her running.
Then for the hourly, 50hour check cost divided by 50 + oil + fuel (upper cruise) + engine fund + prop fund + incidental maintenance.
Then that way you always remain in positive in your account, and fingers crossed keep an aeroplane running!