8 unit opportunity, curious as how to finance best

[Dan White]

Been negotiating and gathering info on an 8 unit building in Clayton, MO, a very affluent area of St Louis.  

Rents are at 5,400 monthly and could be brought up to around 8,000-8,200 with about 70k in repairs.  Great value add.  I'm looking at it to buy and hold myself.  

The seller wants 640k.  I haven't yet agreed to that.  My last offer was 550k.  At this point, he is open to financing part of the deal at prime interest rate.  Right now I believe that is around 5% although he had mentioned 7-8% is what he thought it was.  Regardless, he will carry anything over 400k.  He wants 400k at the sale.  

My dilemma is how to structure this thing with no money down and keep a positive cash flow, even if that means I have to manage it for awhile.  Would it be better to go for conventional or commercial to obtain these objectives? What might a strategy be to refinance this thing best?  Appreciate suggestions from those more familiar with this process!

DW