For the HVAC I would have it cleaned by a professional company, make sure to have the ducts sealed with a smoke sealer product and they thoroughly clean the heat exchanger coils and squirrel cage blower.

I prefer them to have electric stoves, and electric water heaters--two less things for them blow themselves and the other units to Smithereens.Those coiled gas lines do leak from time to time, and the other things I don't like in the units are candles (open candles or religious candles in a colored glass from the local grocery store) or live Christmas trees.

There's also a battery powered trim nailer (ryobi one) which is convenient because then you don't have to mess with air compressors.

I did give Ryobi a second chance with a brad nailer and battery drain auger which work great but dewalt is my brand of choice. 

Second, if they don't know what they are doing they can punch holes in your duct feeds, which are nothing but coiled wire wrapped in plastic wrapped with fiberglass insulation. 

I light a fire in the hot water tank shell, water is circulated by a pump through 2 garden hoses through the heat exchanger coils and back into the tank.My tenants work on things like the heat exchanger coil, but it's upright and about the size 40-50 upright school busses side by side and stacked on top of each other.

Renters dialed the thermostat down to 45 degrees (calling for cold) but cold would not happen.....sometimes the A/C coils inside the unit freeze up into a solid block of ice due to the humidity in the air.Running the unit with a dirty filter might (maybe could) lead to this, but sometimes it just happens.

But if the coil has to be replaced, the cost goes up. 

My quote was for a top of the line replacement new A coil and line set that  is a difficult instalation , or the compressor .

Here is a (600+ sq ft, 1 br 1 bath) solid class B drawing market rate rents (due to location).This is pic from the doorway of the small galley kitchen with a dishwasher, no window coil range, micro and garbage disposal (with backflow prevention between units).Not much counter space.Cabinets are painted over original varnished wood with cheap pulls.No corners and a dishwasher squeezed in, renter provides own fridge (max size = medium to fit the space next to range) and Washer Dryer center about 30 foot from front door.Not the most perfect setup, but the it competes well in the area, and that's all it needs to do.A SFH set up so cramped like this would probably be a loser.