I rent to young professionals that are in their first place on their own and they call for light bulbs out, can't figure out the thermostat, locked themselves out, etc., I don't need another thing for them to call me for!

About 3 months ago I put in new electronic thermostats perfectly wired and leveled replacing the old ones.The upstairs is working fine.

A modern digital thermostat will sense that the efficiency is dropping up and tell you that its time to change the filter.

If this is a 2 story home with 2 thermostats then electronic dampering is a cool feature to have. 

More reference: http://www.appliancepartspros.com/questions-for-ge-oven-thermostat-assembly-wb20k8-ap2623073.html

I am trying to find a thermostat with a pin code that needs to be entered in order to adjust the temp.

One advantage is each room has its own thermostat so you can turn a room off if you arent in it (ie bathroom)

Also make sure thermostat and appliances are easy to use.  

Thermostats, towel bars and other bath accessories, doorbells, mailboxes, house numbers, pull chains, closet shelving, garbage disposals, light bulbs, final cleaning, and all of the other little details can all start to add up on you if you don't plan for them. 

You could get a normal thermostat for $20-$30 or a lower end WiFi Connected thermostat for $60.