1 problem with seller financing the purchase price of a park to a buyer is that the buyer may be lured into agreeing to too high a price.
This is a trap.
Come some years down the road unless you've done serious value add your attempts to REFI out the seller will find you have not created enough appreciation to cover the too high purchase price (balance). This trap is high risk if yoiu agree to a short balloon where it may be impossible to REFI without bringing serious cash to closing.
Now,, it can turn into a strategy at balloon time and no bank will finance you, to force the seller to take a hair cut on the balance due or to extend...
I would go over this scenario with the seller, do the math right in front of him and work out a selling price that does show it should REFI out at some point in the future. I would use 70% LTV REFI terms as well not assume optimistic 80% in your model.