I wouldn’t think this would necessarily be the correct solution in a condo unit.

I found this property through my network of local real estate professionals and direct outreach.

We are in a similar boat, however I have not yet found a solution for less than $5K even for above the ground.

You don't have to spend a fortune and buy a house directly on the beach to do well, but your starting price point for a viable option in this market is around $350-$450k for a solid 2-3 bedroom oceanfront condo or around $500k for a 3-4 bedroom house within walking distance of the beach. 

Private lenders or even creating a small syndicate with a group of investors might also be worth exploring, especially since you can point to your uncle’s track record and your own experience managing rentals as a selling point.If timing is tight, you could look into a bridge loan to secure the property while figuring out a more permanent solution.

There's a few different ways to focus on a lower rate such as with house hacking and making the investment your primary residence, requesting a significant closing credit from the sellers for standard rate buy-downs (which I've had great success with recently), or utilizing lenders who offer short term rate lowering solutions such as 2-1 rate buy downs (again, ideally paid for by the sellers).

Is replacing the HVAC with a completely different unit the best solution?

You might check with your Self-Directed IRA Trust Company/Custodian, they might have a brokerage connection capability where you can setup a brokerage account tied to your IRA, then you can with that brokerage to invest in various public market investments, in some cases stocks, mutual funds, ETF's, treasuries, and CD's.

For your scenario, a DSCR loan could work well if the property’s projected rental income can cover debt payments, but most DSCR lenders don't include renovation costs directly.

And if it were set up where everything went through a title company so the buyer pays the title company, they manage the buyer's loan (from me), and the title company ensures my mortgage is paid without my direct involvement... what parts of the transaction would feel risky or not worth the headache to you?