Would you recommend an entire repair on the garage wall and roof or is it possible to just remove the mold and add some more coating to the roof to prevent further leakage?

If there was leakage or sag then walls should have been jacked up to height before stud repair.

The broken vent pipe is a huge safety issue -- CO2 leakage can be deadly.  

Some water leakage happened and black mold appeared.

If you have a brand new roof, that is a selling point and even if there is signs of previous leakage, the inspector will pass it fine with a new roof. 

You need a plumber to scope the lines as apart of due diligence even if sewer is cast iron all the way to the street.Usually they will charge xx number of dollars per toilet per unit.Galvanized rusts from the inside out so even if you replace all plumbing in the building you can have low water pressure from the galvanized main and also leakage into the ground running up the water bills.Engineer for the city water department should tell you for the area what the average usage of water is per number of tenants that typically occupy that square footage of space.

H2O leakage.

Commonly, with multi-tenant buildings, there is some leakage in NNNs, even if it is things like tenant pays for maintenance contract on RTU and any repairs up to $5,000.  

My tenant reported water heater leakage on 14th Feb.

It has to need to be replaced and that is only if there is leakage and damage.