I agree with everything so far but thought I'd mention that combi boilers are far larger the a standard ones. New wall boilers can take up less space than a single cupboard, the same with the newer floor boilers. Combi boilers need a wall of their own!!!
Get 2/3 other opinions, you may find there isn't a problem at all, but please got to a recognised and reputable company, some of the 'one-man-bands' might be the same as that 'John Wayne' look alike who came recently.