what type of combi boiler do i need?
Insufficient information. Get a gas installer to do a survey and work it out properly for you as any other answers given hear will at best be an educated guess. Apart from anything else sizing a combi boiler is not only dependant on the size /heat loss of the property it also requires some ideas of domestic water usage. Ie a 3 bed house with one bathroom and 2 occupants may just as an example require a 24kW boiler but the same property 2 bathrooms with 4 girls sharing may well require a 35kW boiler to cope with the likelyhood of both bathrooms being used at the same time.
Where? how well insulated? the climate makes a difference