Is there a time limit to pay it back if I have to.

However Boulder County still allows nightly rentals, for now at least (with some restrictions such as needing a license and limits on the number of nights per year).

My rent is $1,750/mo, but I think their limit is $5K.

I see that Oahu basically requires you to be in one of a limited list of properties or condo buildings.

After getting quotes back from my broker, I am being told its actually 25% down and the best rate on a non-owner occupied investment property at this time is 4.125% with a limit of 4 units total. 

A problem with the selling options is selling and finding another property to purchase within the 1031 time limit.

The interest you pay to your bank on HELOC can be deducted as investment interest, with some limitations. 

But scaling up that business of finding those deals, managing the rehab crews, selling it, etc takes a lot of time and soon you find yourself limited.

As long as you have money to buy and maintain them, there is no limit.

I think after covid the limit went up to $75k.