New Heating System

I am restoring a house back from 2 flats to one house. House was built around 1825 and converted into flats in 1950's. Ground floor flat has ch/dhw system, Vaillant combi boiler, all installed about 15 years ago. Boiler now has slow pressure drop, not know if it is the boiler or the ch circuit, this will need diagnosis and fix. Main question is "what is best option for the recombined house"? Separate new combi based ch/dhw system for upper floor? System Boiler combining new ch/dhw in upper floor with exisiting system in ground floor flat? I get different recommendations from different people. Told a system boiler would need to be about 40Kw, don't know output of existing combi, it has been very fit for purpose.