Mega damp gable end - how to tackle

[Betty swollocks]

Hi all,
I am about to buy a house from a family member but there are massive damp issues:
The render is very poor - cracking associated with rusting / expanding wall ties / failure, the cavity wall filling is saturated, I've discovered the overflow from a water header tank has been passing for the past year with the overflow pipe ending in the middle of the outer leaf causing water to run down the outer wall & to fill ups the cavity rotting the wall ties & seeping out through the damp course to the outside.
As you can guess, there is very severe damp on the inside walls, so much so that wall paper is hanging off with black mould everywhere.
If I firstly repair the water leak, extending the overflow away from the house, remove all cavity wall filling & any debris bridging the gap, identify & isolate the existing wall ties, replace the wall ties then re-render the outside.
Once I also remove all affected plaster internally, will the walls eventually dry out, and if so, will the above usually cure the damp problem, or is it a case of once damp always damp?
Can I do any more?

Any guidance from you all would be really appreciated.

Al the very best
Dave - Betty swollocks

 

[foxhole]

It will eventually dry if the damp problems are addressed but will take many months .