I’m an investor in Atlanta Georgia  and I’m used to getting rehab quotes from GC’s that are between $35/sqft (not too much walls need to be moved or roof or anything) to $65/sqft (everything needs to be done) to $85-$100/sqft (new construction or additions).I read about and have ran into other investors who say they can do a full rehab project for about $15/sqft !!!!

I once swapped out a vanity without realizing that the new vanity was slightly smaller than the one I took out, so the wall had to be patched around the entire thing.

Additional units in rentals and removing walls in flips.

A shell, as in just a floor, walls and a roof?

Anything special to look out for aside of the obvious damage to the lining / walls, plumbing?

I have a identical fireplace lay out in my home, wall to wall, raised hearth  (much nicer brick) and have not used it in 25 years.

The obvious......what permits are needed, what work has to be redone, how much opening up of walls/tearing out has to be done, the process, etc. 

As others have said, YMMV depending on what it actually currently looks like and your market, but for me renovated kitchens help sell rentals that are otherwise "idiosyncratic"  walkups with very small bedrooms, slanted floors, uneven walls or sometimes all of the above.

Oops, we overlooked that wall.

You have to be extremely deliberate in how you offer up information so that you don't scare your seller off with unfounded worries.So many BS'ers out there, and many of them hit a wall.