We really can't tell you anything based on the information offered, it depends on local laws, costs, rents, materials, permits, etc.....

They haven't been a maintenance issue other than the glass shades once getting broken which I was able to replace with used shades from a used construction material store like Habitat for Humanity.

For affordable housing, the average construction cost is generally lower than market-rate projects but still substantial due to various factors such as labor, materials, and compliance with affordable housing regulations.Affordable Housing Construction Cost: The cost to build affordable housing in Oregon ranges between $250 and $350 per square foot as of 2024.

The first quote I got was $1,200 - mind you the materials are about $100 - and it's easy to do - could do it myself but have other projects where I'm focused.  

Cost savings over 30 years is irrelevant as it's unlikely to materialize. - No one knows what will happen in the future.

I'm thinking 7 years + to trade equity, 12-15 years for material wealth generation. 

Construction materials: Frame, no basement, roof: asphalt shingles, Floor: Carpet on stairs and bedrooms, LVP: Kitchen and living room.

There is not much material out there on marketing specifically for seller financed notes.

I believe the one good thing about that spreadsheet is that it can be adjusted by the State you live in. because costs are different in every State the estimate sheet i believe is based on a multiplier for the corresponding State, as most cost estimating books are based. that program does break down a lot of information for you and i think it is a good program, one thing you may want to check is if the program updates every year with current multipliers for the States ( program is probably based on a starting average value for each thing and then the multiplier changes as prices for materials change) and you want to make sure it does that.

You eliminate the STR loophole which allows adjusted material participation qualifications