There are numerous ways to structure it, the most common is a 1-5 year balloon payment where you'll pay them the balance you owe by refinancing with a bank, but you can get as creative as you want with it!  

The other unknown factors (not listed in your post) are if any structural framing, roof or other big ticket items are in need of replacement as well which could add to your costs per SF.For a real and accurate answer, best to get 2-3 contractor bids.

Year of the house does not define if the sewer line was ever replaced the date that structure was built.

While we won't give up, I am wanting to explore the option of purchasing land to build multiple glamp sites, small structures and eventually a small site built home (I know this can take TIME). 

Basement storage is very uncommon in this area and would be a continued nuisance for us re: pest control, utility replacement, and addressing the work we have planned on the building which includes structural work in the basement. 

I am seeking recommendations for a structural engineer in the Chelsea, MA area.

What’s the best way to structure this deal so i can see some cash flow?

Last one is - How to Analyze a Wholesale Deal in Real Estate by Simone He- Also from Udemy.com, If you're looking to understand the 70% rule, what an MAO is or how to structure a deal depending on if the buyer is a flipper or a buy & hold investor, this one is gold. 

Structure depreciation is simply the value of your structure split out over 27.5 years.

What other ways could we structure this where it makes financial sense to me (not trying to make a ton, just enough to recoup costs and get me a better return than a bank) and helps them by getting this house?