Natural Gas wall mounted heater, unvented / Need another source?

[Mike Nicholson]

A bit of a novice here so please be gentle. I own a 1100 sq ft house in SC that currently has a Natural Gas wall mounted unvented heater for its primary heat source (1 unit). I have used this for 4 + years with no faults / failures and heats the house nicely. I am recently between renters and had the gas man in the house to turn the Natural Gas back on since the previous renter had turned off and out of their name. Gas man tells me that this is not supposed to be used as a primary heat source and he really is not supposed to turn the gas on unless there is another heat source in the house. Is this some sort of SC code issue?   He does turn the gas back on however this has my interest and I have been researching. Do I need another heat source? If so, I am so confused to the mass amounts of options that I am seeing. Most homes in this area (South Carolina) have central heat & air HVAC with duct work. This house does not currently have any duct work. From what I researched to put in HVAC system with ducts and all would cost me in the range of $6,000 to $9,000! Yikes. Since there is already gas in the house should I buy some sort of gas furnace? If so what kind? Is this vented? Not vented? I am just thoroughly confused at this point what to do, if anything?? Any help or recommendation would be greatly appreciated. As I stated I am a bit novice, only have 2 rental properties in my empire. Thanks so much.

Mike