20 year amortized Seller Financing

[Dave S.]

A little background. I am in the process of purchasing 3 properties from this seller. I've mentioned to the seller that I would like to get more properties off of his hands (he is retiring) but i do not have the funds to do so. I've since spoken with him and he seems to be on board with playing bank for a couple more properties as he has at least two fully paid off. My question is, how do we go about doing this and ensuring everything is legit??? Is it as simple as drafting up a promissory note with the agreed upon payment schedule, getting it notarized and paying $150 or so dollars to transfer the deed? 

As i would eventually want to get him out of the loop and refinance (if it makes sense and when i built up enough equity for 20%), is it as simple as refinancing if i was with an actual bank? Does the seller have to hold onto any of the money (my equity) for the time when I want to refi?

Thoughts / concerns / etc???