As an alternative to a fireplace unit, you could use a recessed fan forced heater. King makes a nice one called a pic-a-watt (
PAW series) that has adjustable 250 watt taps from 250 up to 1500 watts and you could set it to what your setup can handle. They also have a squirrel cage blower and are very quiet. Could use a built-in thermostat, or better, install a remote one if you can. We have two of them in our TT (bedroom & bathroom).
If it'd work for you, King also makes a kick-space heater with a pic-a-watt element and has a squirrel cage blower (other brands have a noisy propeller fan).