Require cashier's check or money order before any keys are exchanged or any lease signed.

I'm not sure if your aware but real estate has many benefits that you don't see right off the top, things like depreciation that can help shelter the income you make from it, and using 1031 exchanges to shelter the capital gains as well, you should consider all those things when making your decision cause it might turn out to be a little different than you expected initially.

Potentially ruining your credit score in exchange for a small loan.

In a tankless unit the minerals will plate-out on the inside of the heat exchanger piping so they have to removed with acid.As with most things;  the "short lifespan"  of tankless water heaters is always due to poor / no maintenance.  

Maybe you guys can take a look and possibly throw out a few ideas.I can't offer my own real estate thoughts in exchange, by the way.

I am in the middle of a 1031 exchange and will be buying another 4 or 5 properties within the next 3 months. 

But @William E. rightly recognizes that a unique opportunity has come your way through that walk through couple.Normally a 1031 exchange on this project would be difficult to defend because the timing and process would look awfully like a fix n flip and those do not qualify for 1031 treatment.

The process is precise but not difficult. 

I'm currently in a 1031 exchange.

Otherwise you can simply state that his daughter is an adult and he has no legal right to interfere as the contract does not involve him if he tries to challenge you.It's easy to get wrapped up in the personal side of this exchange, but don't let that change your approach.