The duct work seems like a big variable so let's say $8k as well.That's $18k in rehab costs before any minor things like flooring / painting / fixtures.Do these numbers seem egregiously off?

If I painted the unit before they moved in, and one wall has stains and smudges all over it, they get charged to have it cleaned/repainted. 

This is a good example of how paint and finishes can add good value to a property without a major renovation.Drain venting is definitely an issue in older houses. 

The economy of scale for 4 vs 6 unit is realized primarily in the exterior paint and roof costs per door, which admittedly may be significant, but probably not a deciding factor.Hope that helps!

When I buy multiple things on a reciept I don't know how to entet different amounts into what category.example: Buy paint, a tool, and all the supplies to put on a roof on the same reciept.I assume all the material for the roof goes together but in what category?

It's not in too bad shape, but was a smoker's house and hasn't had any cleaning done in years.Based on a quick walk-through I'm looking at internal painting, minimal external painting, carpet replaced (or removed to get to the hardwood floors where possible), a bunch of minor repairs to things like gutters/eaves/etc.In a perfect world, this relationship would continue to care for the property as issues occur in the future.

The main 1st duplex has been painted on the outside, the lower level has the new windows installed, both units have brand new high effenciency furnaces and a/c, which the upstairs never had duct a/c always window units.

Graco airless paint sprayer. 

You can then paint the drywall to achieve a variety of looks. 

As long as you are not eating the paint chips with lead in them  you are typically fine. https://www.epa.gov/lead