If you do, just make sure you have all paperwork for current leases, sec deposits, from tenants for past 2 years. more than likely you won't have the verification from tenant that says they agree to what's been paid, a Estopple but honestly come hell or high water if you want that building your going to take the opportunity now that gets you the property and deal with any tenant changes that may come up if they remain.

I'm okay with a piggyback loan to support the remaining down payment (80-15-5).7.

Currently my father n law owns the building and has $125,000 worth of debt remaining on his loan.

There are a few more mobile homes left on the street, so as each one sells the remaining ones seem to appreciate incrementally.  

How would you calculate what the balloon payment would be after 4 years.... is it solely what is remaining of principal from the amortization schedule? 

I'm not sure when the bank is satisfied though. 

This property has been rented out to a great rent to own tenant so I'm sure they will take care of the remaining issues if there are any.    

IIRC, if you take out a personal TSP loan and you don't pay it all back by the end of the tax year, you have to pax income tax on the remaining balance.

@Jonathan Ramos, @Art Herbert is suggesting that you structure the deal to finance the property with a standard mortgage at 80% LTV  (which covers the sellers existing loan) and ask the seller to hold a 2nd (carry the paper) on any remaining balance of purchase price that you work out with the seller.

If I had made a reasonable attempt to resolve the problem through the enforcement of the rules and the tenant still was not satisfied I would likely look at letting them out of the lease.