New to this in desperate need of advice

I am in the process of tackling my first deal!!

I have two properties that I am thinking about buying for 45K/ ea. They currently are both rented to Section 8 Tenants for $750/mth. With a few moderate upgrades I can bring the property value up to 65K within 2months.

The seller wants me to put down 30% (27,000) and he will carry back the mortgage for 63,000. I need to know what you think is the least amount of time that I should accept for repay. 10? 15? years? Is this enough leverage for me starting out?