If a private lender gives you $50k, they generally don’t want to take partial payments and they’re going to charge you interest on $50k until all of that $50k is returned.

At move out we charge $125 for the use of the locks.

Not only is it reasonable for him to charge you $50 to check you out, you should also spend several hundred on a title search to make sure he can actually sell you the property. 

So I'm thinking of setting a fixed monthly surcharge for heat and hot water, plus a charge for the 'free' laundry I provide.

If so, what prevents them from continuing to pay the rent and never paying the plumbing charge?

If you allow pets, charge a pet deposit and fee.

I will charge the buyer as much money as I can, is that fair?

What they charge commonly comes out to $200/hour for labor.

You could also charge them for it and see if they give you any pushback. 

If so, I'm worried about if I could find a hard money lender that would be interested in such a small investment and how it would affect cash flow because some I've talked to charge 9% interest.