That more than likely will be a big deal...and like you said, the stink pipe issue becomes huge...especially if you have a large opening in the wall right behind the new sink location, with no side wall in sight. 

Would a new exhaust system, for example, be depreciated rather than expensed in that tax year?

You do not want deteriorated OR incorrect type of pipe and all fittings, crushed flex connectors, cross-threaded connections, etc.

Lots of interior plumbing that we did ourselves (frozen burst pipes).  2 Kitchens, 2 baths, some electrical, Rugs  Garage partions,  work and it cost approx $40,000.  

The subject property is a condo in a desirable costal market that was damaged due to a pipe bursting in one of the walls. 

And by the time it is all over with, and the eviction process is long, we are absolutely exhausted!

Exhausted, frustrated, and running out of hope so any help would really go a long way.

Most failures of cast iron pipe will be considered capex.

I had a pipe dream that I'd possibly use them for a new build once I became a more seasoned investor.

We had old cast iron pipes in a previous property.