If it's a modern, well-insulated house, it will probably be enough.
Are you planning wall-mounted panel heaters?
Unless your living room is a bit on the small side, I'd recommend a heater at each end or each side, it will heat the room more evenly.
If you are in Aberdeen or somewhere chilly, you could up the power a bit, to 1500W in the bedrooms, or have a portable plug-in heater (preferably an oil-filled radiator which IMO is the safest electric heater) as an addition.
Most electric heaters have two elements and can be switched to give an output of (say) 1kW, 2kW or 3kW. They have thermostats so will turn themselves off when the room is warm. I am not keen on the ones with integral timers, because they are the component that most often goes wrong, and the ones with a motorised dial whir or hum audibly as they age.
If you are mostly at home during the day, consider storage heaters, but up the power as they are not very controllable.
You can buy simple electric heaters at giveaway prices at supermarkets or Argos, especially in Spring, so if you're not yet certain, you could buy cheap ones for your first winter and possibly upgrade later. They usually come with wall fixings which obviate the risk of falling over. In a child's room you need a wire cage.
The price of an electric heater has absolutely no effect on its efficiency or heat output.