I think she would take a partial cash offer, if we could pay off the rest in a short amount of time.  

Short-term rentals might fall under Loudoun's new laws about AirBNB-type rentals, which at this time require only registration (taxation and permits likely to come in the future).

Tim, I can provide you a contact that I've used to look at short-term rentals in Miami.

I'm in a career field where I am likely to have multiple NCR assignments, and have many friends and colleagues there, so I should have some advantages finding quality tenants (possibly myself if I catch another assignment in the area).Budget: I'd prefer to keep it to a no down payment VA loan, which caps me around $680K, but I could put some additional down if i find a particularly good deal.Area: I'm targeting a SFM home  or townhouse in Arlington, Alexandria, or Falls Church (short commute to Pentagon and FT Belvoir), which I can rent easily to upper end military families or local professionals.

As you acquire more you may need to up the number but in short, no there is no true formula. 

If you are happy with an annual lease, that should be a pretty safe bet in most places, but if you're talking about short-term rentals, make sure you're somewhere between Kissimmee and Davenport in a neighborhood that permits it.

If you have any equity anywhere you should easily be able to find a short term loan for $5-10k.

You may want to check on your states short term occupancy laws.

With delayed refinance you're able to refinance after 4 weeks and pull your money out therefore you can buy more properties in a short period of time to scale quickly.

But if she cannot afford 50-80 per month (which seems very cheap for a 1 acre lot by the way) then you will be having rent issues shortly.