Contract for deed, 10 unit apartment building

[Clynt K.]

I'm working on buying a 10 unit apartment building using seller financing. Does anyone have any spreadsheet formulas for seller financing? We haven't agreed on terms yet, but I'm thinking it will be 3 years with balloon payment due at the end of year 3. When determining the interest rate, is it common to have it amortized for 25 years like a commercial loan?  Any help would be appreciated.