Complete novice question

[Zachary Petrak]

I'm in the learn-obsessively phase, but sometimes simple things need explained to me. 

Scenario:

I found a triplex in my area that I'm interested in. The potential seller is someone I know and doesn't do much with the property. The word is that he may be interested in parting with the property. Only one unit is being rented. The other two need rehab, some of which has already been done but the process has halted. 

My question:

If I were to seller finance and then raise private capital for the rehab, how would I pay back the lender loan other than chewing through income from the rentals since I would already be paying the seller on the finance? Especially if the lender wants a balloon at some point? 

 Maybe the property covers enough to pay both, I haven't run the numbers yet as I haven't talked to property owner. This is very preliminary, but it's a basic concept I need to understand anyway. 

Thank you (from a rookie)