I'm looking at building duplexes (out of metal buildings, that look just like houses) and renting them out.

If you have met all the terms of the Land Sale Contract, the deed should be recorded by a title company and your obligations under the Land Sale Contract should end.

Everyone seems to just tear the slate off of them and go with a metal panel roof anymore .

Welcome to Bigger Pockets, love the concept, my wife and I have had some interest in doing something kind of in the same vein, small commercial, maybe some shared office space.Its not something we have done, but I think its an interesting niche to do some sort of small commercial office sharing/storage/ light manufacturing (carpentry, metal fab etc.)I really like the multiple businesses at a time idea.

Now, there is about a 10 yard long metal covered drain in the street right in front of the house.

After I transferred funds from my traditional IRA to a self directed IRA LLC, through kingdom trust,  I then was able to invest in real estate, precious metals, stocks/mutual funds, crowdfunding investments and the lists  of available options are impressive.

I have never heard of Ductwork having to be replaced the system having to be replaced, yes. if the existing Ductwork leaks then look into sealing the joints with duct mastic ( do not use the grey duct tape) or us the silver duct tape. if the ducts are in the attic and bare metal try getting some duct insulation and wrapping them - just a couple of easy solutions.

Metal roofs are another option, and look really nice.  

The expansion tank is a small metal tank, about 8" around and 8" high, that you plumb into the cold water line going to the water heater. 

I have done a unit under 200 square feet that met all the requirements.