In the City of Philadelphia, you will also have to have an extra electric meter to serve common areas, such as lighting and fire alarms. http://www.biggerpockets.com/forums/84/topics/7504...Since you are doing a gut, now would be the time to get the ductwork for both supplies and returns into place.

@Heath D Wallace The first thing that strikes me as odd is your explanation on the work being nearly complete, yet no duct work followed up by your second post that the original contractor is completing finish work while the HVAC contractor you selected is completing the HVAC scope.

Brush up on building code requirements: in basements you're looking for egress windows that are 5.7 sq feet or more openable (3ft by 3ft casement style windows work best), and ceiling heights 6ft 8 inches or greater (as little as 6 ft 4 inches is ok under ductwork and beams though)3.

How about the HVAC and ductwork system?

I've got a few properties that would really benefit from updated heating/cooling systems. 1 duplex I have has electric baseboard heat / window AC (Old 1880s property, no crawlspace or basement, no existing ductwork), another I'd like to add central air too, and a 3rd that has old gas wall furnaces / window AC that I would replace with mini splits (again no ductwork). 

We installed a new truss system to allow proper attic space for ductwork, added a metal roof, replaced drywall What was the outcome?

The warm humidity from the lower was condensating alomg the ductwork for the upper.

Average replacement cost is around $5,900 but can be higher depending on the type and installation complexity.Roof: $10,000 for roof shingles seems plausible, as typical ranges are $6,000-$9,000 for standard roofs.Forced Air (Central Heating & Cooling with new ductwork): Your $18,000 estimate might be insufficient.

But they are not going to go into detail about which walls are load bearing or not, which have electical and duct work in them, all of which will greatly impact the cost to implement their design.And if you want full service design to actually account for all the issues that will actually arise, you are likely looking at 20k+ for design work + 5-10% of total rehab cost. 

I agree with the others about a minisplit (similar to a heat pump, but doesn't require any duct work).