Ive just had the very same done as your asking about. A house with nothing in. We used a Glowworm Flexico 30 cx condensing Combi boiler.
Its very quiet, really quick and simple to use. A Rated energy efficiency and the smallest physical size avaliable so it will fit in a standard 400 mm cupboard.

the reason we used that particular one os the fact it is rear venting so we could have it in the floor level cupboard and not on show as our wall ounted units arent the conventional shape.