. $4500 seems like A LOT of money to transfer our digital plans into CAD (the contractor already built-out our design ideas on his laptop's software) and have an engineer ensure we're using the right beams in the right places.

Keep in mind that some people don't celebrate birthdays.  

Unless you can provide your private lender with secured double digit returns, I agree with most, your dad may have to do this on his own, or simply have you intern with him on this deal. 

Personally I would invest in a fund with an 8%+ preferred return with equity split that pushes the realized return to double digits before I would pay off that debt.

I know of a few funds that offer preferred returns, and project double digit returns in total.

Currently I add the guest's last 4 digits to the locks as a code remotely a few days before their check-ins.

A lot of design questions can easily be answered with pictures and digital correspondence. 

About 500 posts a minute of people that never make it into the double digits on their posts counter.

We are able to get double digit returns with very a passive investment.  

So, I am willing to learn and start slow with the goal to have a number of houses in the double digits which would be fully paid off in the next 20 - 30 years so that when I turn around to sell them, I would have a lumpsum that can add to my retirement.