I wouldn't worry too much about the boiler as long as the rest of the installation looks ok. Boilers don't cost that much in the grand scheme of things (buying a house), and you can get yourself a nice Vaillant or Bosch.
Depends where you're buying. A decent boiler can be 2% (at a push) of the purchase price if you're buying outside London.
Having said that, as a vendor, I wouldn't stand for a buyer trying to negotiate over the state of an appliance.
@rodabod started
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I wouldn't worry too much about the boiler as long as the rest of the installation looks ok. Boilers don't cost that much in the grand scheme of things (buying a house), and you can get yourself a nice Vaillant or Bosch.