Seller Financing Deal

[John Lee]

Hello BP,

I am potentially looking at a duplex that an owner is willing to do seller financing for.  

His terms: 10 year interest payments only at 5 percent.  His purchase price would $309,900.

Both units are two bedrooms and one bath and rent for about 1100 - 1200 a month. 

My question: 

What information do I (or you) need to know in order to make this deal work?

Thanks in advance.