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.

I specifically want to expand on Chris's point by discussing capital risk buckets, which help set a target return range for IRR.At my company, they are as follows:Core: Lowest risk, Class A product, in Central Business Districts, ranging from 7-10%+ levered IRR (since you're in development, I assume this is most of the asset class you handle).Core Plus: Still low risk, strong location with potential upside, 10-13% levered IRR.Value Add: Medium-high risk, Class B+ or B-, mediocre to strong location with operational or physical upside, 13-15% levered IRR.Opportunistic: Highest risk, major upside potential, varying locations, 15-20% levered IRR.As for the GP/LP split on promoted interest, GPs can choose an aggressive split, but the decision should be strategic and consider the preferences and risk tolerances of potential investors.

If you are local check up on your contractor regularly. 

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.