The principle is the same. 

I’ve never had to resort to cash for keys, and never will based on principle alone.

After all, if a fellow investor is willing to take unethical actions in a contract or against a buyer or seller, what makes anyone think this person won't also lack ethical principles when dealing with YOU directly? 

In principle, I don't recommend borrowing up to 90% of its value all the time, just because your equity will sometimes become greater than 10%.

Any advice on what systems/principles work best?

I’m looking for a checklist of everything to buy and do - from installing a smart lock to stocking kitchen basics.

I wouldn't be in a hurry to pay off my primary. 7% isn't ideal, but you can get more return in the stock market.

Over that period you could do better in the stock market.

Then taking that liquidy and investing in uncorrelated assets would be my suggestion like stocks or things that are more liquid.

What will it cost to furnish the unit and keep it stocked with consumables?