I've required rent up front for the entire period, proof of funds to ensure they could cover rent 3x for each month they were staying and a letter from their program administrator ensuring that the students were on rotation and were required to relocate after their rotation was over.

Worst case, give them a LOI (letter of intent). 

Hopefully a letter from a law firm will get them to change their attitude pretty quickly.

One of the requirement that is the most challenging to obtain is a letter from the HOA saying that STR (less than 180 days) is allowed. 

Get an approval letter from a hard money lender or a bank and post a non refundable deposit that in the event the deal doesn't go through the money is forfeited to the foreclosing lender.

Make sure there is a person to get your letters and packages, or stop them for some time so it does not look like no one is at the house.

Assume 25% down.  5 year balloon, 20 year amort.  

Find a local multi-family property that is offering seller-financing, use an agent for this as its complicated and you don't want to get screwedWith seller financing you don't have to qualify with income and can likely get a better interest rateMake your balloon is above 5 years, make sure you are cashflowing in the deal, and try and sell in a couple years for a huge profitYou won't be able to replace your w-2, but you'll get started down the path of replacing your w-2!

If you are going to do the yellow letter approach, that's 68 cents each and postcards are 53 cents. 

I do not like taking over debt with balloon payments coming soon or with variable interest rates either.