This will provide a roughly 10% nominal return likely over the next few decades as it has averaged that since 1900.He says it's simple and other than his son Howard, the others have little interest in investing/finance, and the sp500 index has outperformed >98% of professional money managers over any 20yr or greater stretch. 

ONce you put a rental on your return and it has an accounting method in place, you can't go back and change it. 

It sounds like you've put a lot of work into this renovation.First, let's break down your options:### Selling the Property- You've estimated a potential profit of $58k, which is a significant return on your investment.- Selling now would give you immediate cash to reinvest or use for other purposes.### Renting the Property- With a projected rent of $1450, you need to calculate the Debt Service Coverage Ratio (DSCR) to ensure it's viable

Average annual return from all other investments with him is 1.8%.

First, your return comparison is likely not an apples to apples comparison. 

In another year as interest rates come down, and the boom of cabins that came online over the last two years starts to get sucked in, I think there's going to be a return to solid and steady growth in demand for new cabins.I love my numbers eithe way though. 

I'm negotiating a 7-8% return for them, and I will make interest-only payments until the loan matures (I'll ask for 2 years).

For anywhere you decide to invest in, paying all cash is going to net you a good return.

Depending on how long it will take to complete the remaining work and get the property sold, you could also try to find a larger general contractor; one that could self fund the work in return for a higher payday at closing.  

All they need is balance sheet & income statement to do the tax returns.