Put in nicer appliances, some modern tech tools such as smart detectors and thermostats.

The thermostat for the entire house is in my apartment and I make sure to seal up all windows and doors before turning the heat on.

Thinking of things like keypad door entry, thermostats, fire detectors, water leak alarms...

I'm going to add the new Nest thermostat devices as I retrofit one of my buildings for central HVAC.

Utilities will fluctuate also based on occupancy, pool/spa on timer, owner-controlled thermostats, is garage a/c, if its only 4 beds it could be single-story, how old is the home (energy-efficient windows), how many square feet, how old is the a/c and is it the correct tonnage for the property etc....When purchasing these, that information should be readily available via the seller or property manager.

Because this is a student rental and so is going to utilities included, I was already planning to get a smart thermostat and smart locks. 

They each have their own thermostat so you can control rooms individually.

Should I install a WIFI thermostat and a Ring Doorbell to entice tenants?

Hi Whitney,Talk to an attorney local to the property.You might be able to get a financial judgement (trebled).Step one might be to get three bids on (comparable) replacement systems.American Standard, Bryant, Carrier, Lennox and Trane are names of some furance and A/C (HVAC) manufactures.And realize a new furnace A/C (HVAC) might force you to change the thermostats too (or not).Trying to figure out costs on this yourself (with any resprayed insulation up there) might be pretty tough, (plus you have to buy whatever new refrigerant is required in your area--and the new stuff can be very very expensive).You might be able to pick the brand you like best, or the one fastest installed vs the cheapest...ask an attorney.Also, in a financial judgement, you have to collect...lien their wages or bank accounts (to the extent you are allowed to do so by THEIR Locality).

So you have an air handler that looks like a regular forced air furnace and it has an evaporator coil in it that gets cold or hot depending on what the thermostat is requesting.