I’ve bought three old properties now (one at a time!) and all of them have had surveys that basically say “My God! A hovel! To be expected for its age but barely habitable and only a fool would do this”. It’s the survey’s job to put the wind up you.
You want to take each red flag in turn and weight it up: our current house is 1900 and red flags included “gas hob in the kitchen, untested” – because it’s not the surveyor’s job to test it so they have to red flag it. Versus real red flags like “a literal hole in the roof and a bucket underneath it to catch the water”.
As pp said about damp course injections (a con!), old houses have specific ways they were built and need maintenance to match. Damp in my old houses has been from:
Blocked chimneys, fixed by clearing them out, capping the chimney, etc
Rotten window frames
Too-small drainage pipe on flat bay roof so it got clogged and puddled
Missing lead flashing on the roof
Fireplaces removed and chimney breast closed up without ventilation thingy (old houses like to air!)
Blocked air bricks
Paving built too high outside or sloping against the house – old houses tend to have air bricks that vent under the floorboards, these need to be kept clear, not built over, and create a through draught from one end of the building to another, under the floorboards. They’ll also likely have a piece of slate or similar a few bricks high on the outside: often people pave up and over these and the damp begins; can be solved with a French drain - cutting away 4” of paving nearest the house, digging down and filling with gravel
Condensation – new, non-draughty windows installed, fireplaces closed up, floors covered in wall-to-wall, shiny latex paint on walls, and lots of tumble drying. Solved by removing all that stuff and opening a window!
Mysteries! My parents have lived in their Victorian house for 35 years, solved all of the above and more, and there is still one small patch of damp below the bay window in the dining room. Never figured it out but the house is still standing at 140 years old and tiny patches of Mystery Damp, once all causes have been ruled out, can be solved with some Ronseal Damp Paint and an emulsion top coat