She likes the secure feeling of having it paid off.

It means you need to secure flood insurance which is in addition to wind/storm coverage in Florida.

I've built up some experience completing flips and buying rental units on my own – and now would like to start bringing some other investors into the fold to secure deals on a more consistent basis.

Make an official written offer get it under legal contract that states if you give your friend the 10,000 he must sell the propperty for the agreed price and then just do a coventional loan.

My sister can possibly secure a personal loan at low interest. 

If the seller is an owner occupant, he may have an LTV much higher than any investor can obtainThe main benefits to a seller is that the transaction is quicker than a buyer obtaining new financing, and that he may get a higher price because the buyer does not have fees associated with obtaining a new loan and the buyer may pay more for greater leverage or pay more if he can’t qualify for a loanThe downside to both is primarily that the buyers equity can be wiped out if the note holder accelerates the note, further the seller now has liability on a note secured by a property he does not own.

Depending on jurisdiction this can be informal process, or an official round table session of sorts.

I secured financing using an owner occupied fifteen percent down loan.

I noticed the seller provided documentation that none of the units put down a security deposit so that raised some red flags.

There will be ups and downs (like the AirBnB property in Costa Rica where that woman was murdered and they arrested the security guard) but by and large I think it is here to stay.