It's incredible the responses you get if you post a question.  

But what about things like knocking down walls, building new ones, window replacement, kitchen remodel, etc... do i need to hire contractor with a Construction supervisor license?

North/South are Wall, Washington & Harrison; East/West: 12th, 20th, 30th, 36th & 40th.  

You will need to make sure there are smoke detectors and a couple other minor inspections items.Of course, you need good walls, and a dry area so it isn't a wet mess...If you want details, give me a shout.

We're talking about 30 ft of cast iron replacement (9 of which is behind walls between the duplex units, rest in basement), stack replacement, and excavation to find the outside access to the lateral line (underground somewhere so they werent even able to scope that yet).I could also go the route of lowering the offer based on the estimates, but is it worth me doing that work myself if it doesn't raise the ARV and thus my equity to get the money back later?

They tend to leak and when water infiltrates the roof seams the water will  travel down inside the walls and when the water freezes the water will expand and blow the walls out. 

There is a property we are interested in purchasing.The house was recently torn down all except for two walls.  

And an incredibly process at that.

I thought it would be like a piece a cake, but it a wall that I can't seem to scale.  

So, I used a floor mounted, wall outlet toilet, not floor outlet, by American Standard.