It's exciting to know how much business is going to come from this, but at the same time terrifying because I feel like I'm not going to be able to continue at this pace as the quantities get larger. 

@Jay HinrichsRunning your investments as a business is essential- your point of having sufficient cash reserves, cash flow and being willing to make it through good and bad is crucial.  

But here's the thing with an FHA loan with either 3 or 4 units (duplexes are excluded from this), you must pass what is called a "Self Sufficiency Test."

@Stefan Demopoulos, as Jay said, the self-sufficiency test is the reason it wouldn't likely work in this interest rate environment.

I would keep it to two units as most 3-4 units right now don't pass a self sufficiency test and will require much more than 5% down.

Simple W9 should be sufficient 

While the attic distribution wire was sufficient for 30A, the feeder lines were not.  

If you have sufficient # of unit to where the cost of hiring a book-keeper is worth it, then yes, get one.

Having done a number of shower stall tile installs I am amazed that anyone would expect the paper coating on green board would be sufficient to absorb the thin-set to ensure the  adequate adherence of tile to a vertical surfaces vs that of cement board.

@Jake KozulWhile Nicholas is correct that no thermostat is tamperproof, you only want to make tampering sufficiently difficult as to deter it.There are several, mostly commercial, thermostats which allow for different levels of users and allow you to restrict some or all of the functionality.