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.

Add a programmable thermostat.21.

In addition, he told me that there is currently only one thermostat for the entire house, and that it's difficult and expensive to zone on a steam boiler.

Yesterday I received a text stating a tenant did not have heat. allegedly the pipes were banging but no heat was produced so they shut the thermostat off.  

This comes with an Aerus Air Scrubber and an Ecobee smart thermostat and sensor to ensure I can keep the basement and upstairs temps nearer to each other and at least some cooking smells don't permeate.

The triplex has one thermostat for all 3 units.