I know it will be just as much a re-sale consideration in a decade as it is for me currently, but since I see this as a long-term buy-and-hold, the more immediate concern is determining ARV and what it would appraise for when the time comes to back into a conforming mortgage after seasoning (BRRRR style). 

Arthur, I have 2 concerns here.

Seller is willing to do seller financing on one of the properties, but his concerns are:1) Not losing money compared to an outright sale and dumping funds into stock market2) FHA inspecting his unpermited 5th unit.

Without completing all of the above, you’d be looking at significant condensation leading to a mold concern, stains on your ceiling, replacing ceiling and ceiling insulation within a few years.

Supposedly current on rent according to the spreadsheets the sellers agent sent me.The house had a fire in the attic before the previous owner ever bought it, so some of the wood there is charred.The concerns are:-Do I trust their spreadsheet as proof the tenants are current on rent?

I am concerned about the large equity I have in the property, but I don't think there's anything else I can do to protect it. 

I am concerned about cost to a degree but I feel if I have the correct guidance on what would be the smart way to use my money on the house it could be of huge value to me.Any and all suggestions and guidance are welcome and appreciated. 

If you are concerned about lawsuits, I would talk to a lawyer. 

I've got a small business that I'll be working full-time so the runway before my FTE position is up is limited (in re: to financing concerns). 

They are not concerned about making huge money off of this deal and are ok with just pocketing the interest difference and knowing that they're helping up grow our business.