I am planning to use Longhorn Investments hard money loan to fund the project.

You can often make MORE money in the end charging a little less rent by reducing these things. 3.

During this time you are not generating any return from the money invested in the ADU.

Rentals create cash flow, and that's what we want as real estate investors, but you need cash on hand for unforeseen circumstances.The AC needs to be replaced, that's $7K to $10K or more depending on the size of the house and if you have one rental, it will eat all your cash flow, same to replace a roof, etc, or if tenant trash the home, you need money to fix it again.To me personally a nice rental portfolio that is healthy consists about 10 units, so if you have an issue with a house the portfolio can cover the repairs and still healthy, but when starting you need to have cash on hand just in caseYes, there are multiple strategies you can use to hack your way to buy your first rental with minimum cash, but I would start wholesaling or fix and flip a few deals to create that cash as fast as possible

Should I sell (use the money to buy other rentals) our current primary residence in Tampa or rent it out?

Pick a county and look up Property tax sales.  

So I worked 4 jobs in that 18-month timeframe to come up with the money for a suitable downpayment (without touching my brokerage, 401k, etc).Gosh!

Also, how can I estimate the taxes?

The people in the room are the people you do business with they have the deals, the money, the services, the knowledge, the legal help and everything else you need to do a deal.You could try to make those connections here on BP, but nothing beats going to a meeting and talking live and face to face - you might event get to meet Andrew, he speaks from time to time - or all those vendors - we had 40 of them in a room last month just for everyone to come ask them questions.So find your local group - where ever it is - find or join several. 

She's then at risk of losing her money, but it also means she won't do that to another landlord.