depends on your insulation. A 1kW heater is the minimum I would use in a bedroom. With a thermostat it should not be running all the time. A 2kW means you can put it on high to warm the room, then turn it down to half-heat for a regular temp.
IMO Dimplex are excessively expensive because they have a brand name. There are some very flashy electric heaters sold at very high prices because punters have been tricked into thinking they produce more heat with less electricity. This is however an impossibility. Al electric heaters produce the same amount of heat for any given amount of electricity.
I am sure some of the unbranded Chinese imports will not be very good. There is not much to go wrong in an oil-filled radiator, they seem to last for many years. Timers go wrong, and I suppose switches might break, or the castors might break.