When you go to dry wall I would 100% pay the extra money for your favorite "concrete" board. 

We were constantly repairing furniture, broken chairs, touching up walls, unplugging potties, etc. 

We got the spots treated for $750 (seller paid for it), so i thought everything was good.2 months into the rehab I found severe termite damage while tearing out drywall in the spot i had sent my realtor a picture of and it the downstairs bathroom on a load bearing wall which will probably end up costing me thousands.I should've done better due diligence, however, I am not an inspector and thought I was covering myself by getting it inspected/treated prior to buying.

We did some renovation in the kitchen, paint the walls and kitchen cabinets. 

Whenever I see a wall oven in an investment property I start thinking about that difficulty and expense of replacement.

I have seen full gut remodels in the 3 units, but they already have a foundation and exterior walls, which cost a lot of money to erect.Since I am guessing you are not jumping into a $20mm project as your first go, buying will almost always yield better results. 

But it had damage to the walls, and floors, the kitchen was in disarray, I assume the bathrooms will be as well.

The house was in BAD shape...We moved the deck, changed the floorpan, moved walls, new floors, bathrooms, kitchen, roof, painting, a bunch of grading work and exterior stuff, new landscaping, exterior trim work, and removed the family of dead raccoons!!

There is a heater duct in the room, but nothing else.The township inspector has requested that I install vents in either the wall or the door.

If you're taking down walls, or doing extensive plumbing (like adding w/d) they might get the building architect or engineer involved.