Backstory:We rent the ADU out regularly via STR or MTR and live in the main house currently and are looking to size up.

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.

If you are local check up on your contractor regularly. 

Sure, it's 1954... there must have been a zillion cases and whatnot since then.And with Section 199A, you can use either the safe harbor or facts/circumstances to support 162 (frankly, the safe harbors are anything but safe, I'd take my chances on simply arguing continuous, regular, profit motive, etc.).Perhaps DM me.

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.  

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.