The respectful tone of your title drew me in.  

The behavior of the tenant and friend are uncalled for, but don't get sucked into an argument with any tenant.Don't get a gun unless you absolutely know how to handle it and go to a range and continue to practice. 

Another method is physically going to properties on the market in your area and look at them, look to the basic concept of "DUST" in the real estate, after about 50 homes in a band of price ranges, you'll be able to guess, it might take 100.Too bad newbies don't have a "Blue Book" for real estate, wouldn't that be great......Oh, that's a 2006 model Harrison Built in Oak Hills, zip code 65606, ah, it says here the 3/2/2 is worth, in good condition, $212,500.00 !  

Any guesses at a cost range would you be looking at to build a single garage with a loft above to rent out?  

@Russell Brazil much appreciated, I definitely respect your position. 

The old rule of thumb was that a prospective borrower could get a mortgage of about three times (2.8-3.6) times gross household income at interest rates in the 6-8% range.

As long as there are crystal clear expectations and respect.

It is interesting to note that your wife would be a disqualified person with respect to your mother's 401k even though your mother would not be disqualified relative to your wife's plan.Again, these prohibited transaction rules and the avoidance of disqualified persons does not apply to funds that you have borrowed from your 401k via the participant loan feature.

I think you can find some properties in that price/rent range. 

I would respectfully disagree and I believe the two are mutually exclusive...and should be.