storage heaters are annoying unless you are at home all day, as they give out their heat all day, when many people are out at work, and run down by evening. Very many people who have had them hate them. Some pensioners like them, apart from the running cost. You must not put anything on top, such as washing or towels, as they will overhat and the fuse will melt. This needs an electrician to replace. ££.
They are also more expensive than gas to run, because energy from electricity costs about three times as much as energy from gas. With a White meter you might get about 50% discount for the night-time use only but it will still be dearer.
Electrical heaters run on daytime tarfiff are very very expensive to run.
You will also need a hot-water cylinder with immersion heater, preferably a very big one so it can be heated overnight and last all day.
If the existing storage heaters are old and you want to buy new ones they will not be very cheap. Some people will try to sell you magic ones that generate more heat energy than the electric energy you put in, but this is a lie marketing story. The efficiency of electrical heaters does not vary with their cost, though they may have better controls and insulation to delay the daytime heat output, the amount of heat you get out will be exactly proportional to the amount and cost of electricity you put in.
Houses are quite expensive to buy, so £4,000 for GCH on a £400,000 house is not much and you will appreciate it more and for longer, than most other things you'd spend money on. One day you will want to sell the house, and your buyers will look on the absence of GCH with horror.