is it damp or condensation? with condensation I would take the chance, with damp only to tear down and rebuilt if it can be easily rectified. I would expect that to be reflected in price and a survey paid for by the vendor.
Yes, sell it damp free. People are often scared of damp and worry it will come hand in hand with other (expensive) problems. It is often worrying enough to put them off shelling out for a survey as they may expect it to come back with 1000 issues, so would rather keep their money and keep looking!
Depends on the house. We sold an old house that needed other bits of work with damp in situ and a quote for remedial works of about £1600. I think it helped that we got rid of ALL the condensation damp and then repainted any previously damp areas. If you have the time and funds though I think you’d be pleased to have fixed it before going on the market (make sure the work comes with a 30 year guarantee)