Main thing with a heat pump is that the water temperature is lower, so you need a relatively larger radiator compared to sane house on oil, and you may need to leave running at night or on night rate prices.
Steel radiators can last 40+ years no problem if painted and not in damp area. There is no real difference in both types, maybe aluminium will last longer but typically radiators don't fail. The larger the radiator the quicker it will heat a room and if undersized a room may never get warm. Ideally you want an electric thermostat in every room on the wall, controlling the radiator in the room, or at least a thermostatic valve on the radiator which prevents room overheating. Ideally you need lots of zones and good controls. Typically a heat pump is more expensive than mains gas to run so you want a very well insulated house to try keep electricity bill down. Electricity is very expensive and often created using mains gas.
