Re: Water leak between wall and concrete slab
Any professional carpenter or general contractor can evaluate the issue.
As for replacing the bottom plate --- it can be replaced ,with some effort, without tearing down the wall. Temporary support structure to hold up the roof will need to be in place ---the stud to plate connection ( nails ) will be cut --- plate removed and new slid into place --- studs re-fastened to plate.
However, if the bottom plate is rotted in one area it's likely you'll have others that may need attention as well --- especially if they are not pressure treated lumber laying on concrete.. Also, depending there may also be rotting to the studs resting on those bottom plates in which case they will either need splice repair or replacing.
Now, the existing structure may not be suitable to be converted into living space.
One thing to consider is the concrete slab -- it will be cold and will need special attention to insulate.
Also, is the structure built to current building codes?
Having a professional carpenter or contractor evaluate the structure for repair will also provide you with information if it can be worth using the existing struture or does it need a tear down and rebuild.
"" an ounce of perception -- a pound of obscure "