Sash windows in a listed building

Hi. I'm supposed to be buying a flat on the third floor of a Grade II listed building in a conservation area but the survey has come back terrifying. Among the list of things (roof needs to be replaced, wood boring beetles in the roof space etc) is the windows being rotten, cracked glazing etc. Does anyone have any idea of a rough cost to repair sash windows? The vendor reckons she's already knocked money off to account for the work but at £150k for 412ft/38.3m2 it doesn't feel like she has. Also, should I be alarmed by the roof? The surveyor said it had been patched up over and over again and needs to be completely replaced but I've read that's not best practice with historic roofs as it's preferable to replace only what needs replacing. Any advice/insights welcome as I'm losing sleep worrying about it all!