I look for something that is around 6mm in thickness and has a wear layer of around 20 mil.

"Trust Layer" might but I think its more for subcontractor COI tracking.

Older buildings likely have nothing but one layer of sheet rock between the floor boards/joists and lower level units.

I'm also not sure if us rehabbing their unit and having them come back to start a new lease in fall would add another layer of complexity.   

If the wiring is nonmetallic cable (romex - plastic looking outside layer of insulation) and has a ground wire (3rd wire, bare copper) it might be fine to keep.

A GC will add another layer of accontability as well. 

I am looking for thoughts on whether I should be proactive with replacing a 20yr+ old roof (missing shingles, multiple layers, looks terrible) and a 25yr+ old hvac.

That distinction is important for one very important reason, the asset decisions are being made by the owners (or a direct family member) which eliminates a layer of defense for all these decisions.

Tax strategy has dozens of layers.

In the kitchens we found up to 7 layers of flooring.