Best central heating option for 3 bed house/1 bathroom

We will shortly be moving house into a 1930's property which has presently 3 bedrooms and 1 bathroom. We plan in the future to extend the property adding another bedroom and 2nd bathroom. The survey has flagged the boiler existing system (back boiler) is at the end of its life (from what I know its at least 20+ years old) and so i want to get ta new boiler that serves the house effectively as it is now and the extension in the future. So far I've received one quote for: Worcester 30CDI system, with vertical Flue kit, pitched roof flashing, 250 l unvented HW cylinder, 1 x chrome towel rail 600x1000, 3 port control pack, wireless room stat, condense pump/pipe, re route of gas & cold mains Total £4358. This seems expensive considering non eof the rads are being replaced except for the bathroom. The engineer states its going to take4-5 days. Can anybody provide any advice guidance on a) is this reasonable quote and b) the right type of system given the size of house and planned extension in the future and if not potential alternatives to consider. Any help would be much appreciated.