You may not want your heloc tapped that long as the rate is generally higher on a heloc than it is on fixed mortgage rates. 

They would fix my $100 for the next 18 months as a grace period so I have time to put the tax credit money "back in" the full cost of the panels.

Just make sure you try to househack in a desirable area where people will pay a high amount per bedroom. 

Active investing you directly get appreciation + mortgage paydown + cashflow ALL ON A FIXED 30 YEAR MORTGAGE.

I have listed my house for rent for $2400 trying to get a section 8 tenant because of the guaranteed money from the state but willing to lower that amount if need be.

Spouse and I are small time real estate investors, we are looking to do some fix/flips in Texas.

TIAErich  Erich - I would echo other's comments, but also point you in the direction of a Bigger Pocket book, "Estimating Rehab Costs" - It's a 2019 version, so the actual dollar amounts will need to be increased, but it can give you a good starting point for the process that you can refine/update over time. 

I didn't get an LLC but got an umbrella policy and talked with my insurance agent about the amount of coverage on the home. 

I'm hoping to secure a fix & flip for my next deal and I'm looking out of state (specifically in Oklahoma).

If this is a legitimate mold issue: need to get it fixed.