You only have to heat as much as is lost, i.e. replace the lost heat. If it currently has old storage heaters, then what are the chances that the insulation/windows aren't up to spec, either in the walls or roof, if there is one?
The biggest bang-for-buck is to maximise the insulation, then (if it were me) I'd look at a small wall mounted heat recovery vent. (Because once you insulate and stop any draughts, i.e. uncontrolled ventilation) then you will have to address the humidity/ventilation issue). Only then would I look at what heat source to use.
But that's just me