It also has a burner and a heat exchanger.

Would anyone be able to give rough estimates of electric/heat/air bill.

This is a 2400 sq ft 2 king suite cabin loaded with amenities including an indoor heated pool similar to one of mine.  

But say a $10k heating system that should last 25 years (300 months), that alone is $33.33 per month ($10,000/300), even if it was just replaced yesterday since it will need to be replaced again in 300 months.

A month later, not much had been accomplished, and now the workers couldn't work because they had no heat, and we couldn't get any fuel, so they ordered an electric furnace, which was just installed last week.

I would venture to say it has everything to do with the draw of renting a house, in the heat, and not having a pool to cool-off. 

In the mean time, I removed the LP heater and the leased LP tanks, so there is no more gas used in the place (so the CO part of the alarm shouldn't be an issue) and installed a mini-split heat pump in its place.I hadn't heard anything from the tenant but, when they moved out last week, I saw that they had again disconnected the alarm in the bedroom.

They tend to heat up when rowhouses cool off and vice versa.

I consider buying houses with bad roofs, no HVAC or heating only, no central air, old or no appliances, poor paint. 

Heat/electric avg is $150-200/ mo.