How much time do you have and what is your investment strategy, fix and filp, BRRRs, STRs, buy-and-hold will require different hunting techniques. 

I get that this was geared towards flippers, but I feel like the premise may hold true. 

So we (wife and I) are ready to make that final push towards retirement with long term buy and hold properties!!

My wife Jen (a real estate agent) and I are real estate investors mainly interested in buy and hold single family rentals, as well as flips. 

She specializes in what you seem to be after: buying up stable businesses for cheap, improving them, cashflowing and then repeating the process to keep expanding. 

Are you essentially acting like a contractor, but just within an airport environment and selling the renovation while your client holds the lease or is there another process I'm missing?

Considering your situation, holding onto your current duplex with its strong cash flow and exploring refinancing later when market conditions are more favorable and you have built up more equity seems advisable.

That holds a lot of risk in money, time and stress.

Investment Info:Single-family residence buy & hold investment.

Holding the note that you originate is a depreciating asset.