still can't choose heating and air conditioning system - what am i doing wrong?

With a combi and radiators you've got a good heating system already, so the real question is whether you want cooling too. If yes, have a look at an air-to-air heat pump (split system). It cools in summer and can heat in spring and autumn, so you run the boiler less. If you only want cooling for a couple of rooms, a single split unit is cheaper than ducting a whole house. Whatever you pick, get a proper heat loss calculation done on the house first, not a quote based on square footage. Oversized units cycle on and off and never run efficiently.