Plumbing work typically costs much less than electrical work, so I always advise to stay away from properties that have all the wiring pulled out or, if you run into any knob-and-tube electrical - no thanks!

One of my client's is being sued creatively by an adjoining retail condo for alleged illegal or negligent installation of plumbing. 

The 4 new apartments are going to need all of the interior framing, windows, electric, plumbing, heating and also new siding for the building.

I understand the need to use licensed/insured tradesmen for electrical, HVAC, plumbing,roofing and I will easily be able to obtain W-9, copy of insurance.However, my plan for flooring/sheetrock/painting/siding is to use sub contractors that may not be able to produce a legitimate W-9, may not have insurance, etc.Is this a bad decision?

If the plumbing is separate, which it rarely is if it wasn't intended to be.

Could be roof leak, or plumbing pipe leak, maybe a drain.

The home is old, built in 1919 with original plumbing, wiring and two major floor to ceiling cracks in the foundation.

Was taken down to the frame and new electric and plumbing was installed. 

The property is going to need a lot of electrical and plumbing work, in addition, some walls are bubbly so I will have to check for any leaks.

It may also require some boring into concrete and treating the interior walls by plumbing fixtures.You should seal up the exterior to prevent rodents if that has not already been done.