Hello @Gregory Evans, this sounds like a really interesting deal. I once lived in an apartment complex which was really just two buildings across the street from each other with the same paint job and owner with a small shared community outdoor area. In other words, seems like a unique way to increase the value of the two buildings by consolidating them as one entity.
So of course first you'll want to find a real estate attorney with experience specifically in deals where the seller holds the note. They should have no problem drawing up specific terms of the deal to present to the seller. Not sure what town this property is in but you can just call that local County Tax Assessor and they should be able to give you all the numbers on that.
As far as other closing costs, THIS could actually be your leverage! You will both save big money because their is no loan origination fee, no escrow fee, however you will need to work with a title company too formalize whatever your attorney draws up. Also, the seller may require specific types of insurance to protect themselves but ultimately another bit of YOUR leverage should be that he will have the opportunity to foreclose on you if you don't meet your end of the deal.
So for your leverage:
-seller avoids some closing costs (YOU actually save a ton of closing costs..)
-seller has the ability to foreclose on you and keep the payments you've made to that point (and the property he would potentially be repossessing would be totally upgraded on your dime
-seller receives steady passive income which may be very attractive to someone that age
As far as other leverage, the most important thing to do would be to find out more about HIS needs. You can try to find out his goals and shape the deal to help meet them.
I think you could actually get a better idea of how this would work by checking out some of the bigger podcasts videos I've watched recently regarding mobile home investing. It tends to work similarly where you buy and sell for payments because traditional financing is not an option just like in your situation. One of the podcasts I was referring to was episode 208 with Jack Baczek as well as anything John Fedro has put out.
I hope this helped a little.