You also have to have a system in place to vet your guest (which Airbnb doesn't do anyway - all they do is "verify" their identity), create a lease agreement, accept deposits and payments and issue refunds of deposits.

Houston isn't making 4 unit buildings regularly.

Also, Z Wave and Zigbee radios are in sleep mode 99% of the time whereas WiFi radios are communicating pretty regularly.

They are not as scary as some make them out to be, you just want to be certain that you can regularly contribute to the principal of the HELOC/home equity loan (on top of your monthly interest only payments), so you can pay the balance off within a reasonable amount of time. 

But if you are using seller financing and a regular CRE loan, the seller will need to be comfortable subordinating.

It is part art and part science.Many of our rental properties are almost identical down to the finishes and cabinets (and in the same suburbs of Milwaukee) and I get a 10% swing on appraisals - at least 5% in either direction.

Most investors I work with do either a regular refi DSCR loan or, take up to 75% or 80% of equity out of the home through a cash-out refi. 

I would start off by getting several quotes.I can tell you that my regular AC guy charges 40% more than the guy I go with.  

If you are local check up on your contractor regularly. 

It looks like your seller wants to seller finance the deal to 'be the bank' - which can be great - maybe you can negotiate a better deal on interest rates than what is currently going on in the regular market - which would be awesome.As for which home first - I would definitely go for the duplex you are already in.