I am currently working at a plumbing company and pursuing my goal to start my own plumbing business one day.

Things like new roofs, plumbing, and electrical don't have a very good ROI but cosmetic things do. 

Put up a new wall, within that wall is new plumbing to a bathroom.

Add in some expensive plumbing issues that would have to be redone and several code violations, this was another one we decided to pass on.Obviously there were many others, but these were the most memorable ones I could think back on.

Tenant A runs a plumbing and heating company and did some work on the garage himself.

Plan for extra expenses, including wiring, plumbing, and any other unforeseeable.

Home depot, lowes, or a local plumbing supply will all have comparable units.

Those documents also define the Common elements  which the association is normally responsible for insuring and the Limited Common Elements (things like plumbing and HVAC piping, wiring, etc. that service more than one unit but not all) which could be the associations responsibility or yours if they are in your unit.3.

The building had almost all early 1960's wiring and a mix of galvanized and copper plumbing.   

Does the wall have wiring, plumbing or HVAC that will need to be rerouted for example?