I agree with your point about whether you could improve revenue with $10-20K invested, I think that's a framework to decide if it's worth it to keep going or get out. 

One major challenge is having our partners come up with clean and organized documentation of their track record.

I'm curious to understand if it is an area worth investing in.

So i'm in a market with little or no comps and there is no general cap rate for the area. with this in mind how do I figure out what my building is worth? 

There is slightly less risk if these MIL and additions have been done decades ago, versus last year, but still worth checking with the building department and zoning to understand what their enforcement is.

If you have excellent credit, a track record owning rental properties, Loan Amount is over $150,000, you are the sole owner of the entity, and you have the reserves,  Most DSCR lenders will give you a great experience. 

Right now you are probably looking at somewhere around 7% on an investment property, and with your tax situation the interest is probably worth maybe a 1% return to you, so you'd need to do something better than 6% with the money to make it make sense. 

Additionally, the 1031 exchange is worth mentioning though I'm sure you have already thought of that. 

When we have a written record of communication it helps to avoid any miscommunications & the inevitable "he said, she said situation." 

It costs more, but it's so worth it.