Sub floor damp joists

tollytolly

We’ve found a Victorian mid-terrace we’re interested in buying. A specialist damp survey has turned up some work in an internal wall and a disused chimney breast and the estimated cost to fix what can be seen is around £5k. The problem is: the damp is in an internal wall, and obviously needs to be coming from somewhere but as we can’t get access to the sub floor area we can’t see the cause and also the extent of the damage down there.
The living room is 17’x12’ and the dining room (other side of the wall) 13’x12’ and for price estimates, this is in Scotland.
So I have a few questions
1. If all the joists are rotten and need to be replaced, what would the cost be? What is the process for doing this? (One estimate I saw was £100 per joist, and at one joist a foot, would that be in the right ballpark? Re the process, do the walls need to be jacked up or anything, what are the costs involved with that?)
2. If the internal wall is involved, is it possible its support would need to be replaced? (Surely its support isn’t wood so that should be ok?)
3. What other eventualities have I not considered? I mean absolute worse case scenario, what else can be wrong and need fixing?
Any help would be very much appreciated as I am a complete novice in this area.
Thanks very much for any assistance whatsoever!