Sell - Would prefer to avoid this to keep my low interest rate (3.3%) and avoid fees/taxes. 

Cost segregation studies can offer real estate investors massive tax benefits.

Any ballpark idea what percentage our property taxes would go up if we went from 2 to 3 units and how does that get plugged into the calculation?

I am also extremely interested in Portfolio loans, but I have zero contacts dealing with those types of loans.Finally, I have an 'excellent' credit score, no debt at all besides 5 other rentals at 75/25 LTV (all cash flow positive) and a job that I claim over 100,000 in taxes, and have three years of consistent tax returns showing that.

She might not qualify for another loan, unknown it depends on if she can show it as positive rental income but with your plan it sounds like she won't have tax returns in the future to make this work. 

If the Florida property could cash flow or at least pay the mortgage taxes and insurance, utilities ect then they would just have a payment for their house by me.  

I also got the taxes reassessed and cut by more than $7k per year, due to the tax assessment being overvalued compared to my purchase price.I just went through the process of getting a 25-year commercial loan put on the property with my usual lender.

The goal is for each of us to retain ownership of the completed buildings, though we're undecided on the ownership split.Appreciate any advice or recommendations you can offer. give a value to everything. your deferred GC fee which even if you do it for 5% markup I would so you earn something and aren't hoping something happens down the road.

(plus higher property taxes due to the new assessed amount)2- I don't have cash for the rehab yet, so any advice on going about sourcing the 100k?

We know this from historical tax court cases.