Tim I have an Old 1800 sq ft 2 story home and I just had to replace my boiler in Nov after it developed a crack in the jacket.I also had to tear out the old supply trunk in the basement. It had been originally meant for steam and carried too much volume of water for the new boiler that was too small for the house anyway. This whole job was done for $6000.00 by a longtime friend and damn good plumber. Im sure he didnt charge me the full rate as I helped him with some of the work.
Im sure that your job will not be cheap but what I learned was that it is very important to get a boiler that is the right size for you space, if even a little bigger than is needed. Also how old is your current boiler and what is wrong with it? Most boilers should last 30, 40 even 50 yrs with proper maintenance and installation. A boiler that is too small will not only be inefficient but it wont last as long as it should either. Your plumber should be able to calculate the right size for you based on the number of zones you have, the number and size of radiators, how well insulated your house is, the sq ft and the volume of water needed to support it all.