The home need some instant patches - roof leakage, but is currently rented.I assume the best option will be to close on it, get the insurance and fix the leak.

Additionally, his car was leaking oil in my garage and my contract specifically requires that the garage floor is clean and no oil leakage.  

That will stop the air leakage.

There was no evidence of any leakage of combustion gases or hydrocarbons.

With water it can be 100 different things.If you can demonstrate the leaks to the water department they will usually do a bill correction adjustment for most of the leakage. 

Too many complications can arise (insect damage, rot, leakage, shifting off footing, and ultimately collapse if not consistently monitored and inspected) DO NOT buy the house without getting a proper inspection done on it first.

First vent to the outside for the fan and make sure there is insulation in the attic above this room and no air leakage into the room.

There may be a leakage so the stains.

I bought one across from a gas station and prior to getting the Phase 1 in talking with the owner of the Environmental Services company he said he was confident based on the grade of the land, there would be no issue (because any spill/leakage from the gas station would head in a different direction - away from my building). 

This leakage is zero percent the fault of tenant.