I assume that you had to add the ductwork when you had the ac installed, as you mentioned radiators. I also have a radiator heat/ac cooling home. Our ac air handler is also in the attic like yours. Our basement has no ac also like yours. Couple of things to ask about. Often these ac retrofits are not ideal design, but practical design to work within the existing home as it relates to ducting and registers. Often there is not a supply and return air register in the rooms, but a ceiling supply register and undercut doorways for return air flow. Our system has a large return air vent in our hallway ceiling going to the attic. Ask your service tech if the duct registers can be adjusted to force more air downstairs. Ask if there is good airflow overall and good return air to your system. If not can they suggest any changes? Were your doors upstairs undercut about an inch for return airflow? Can they add some changes to the ducting to get more airflow downstairs? Ask if the unconditioned basement is contributing to your cooling load on the first floor. Are you running a dehumidifier in the basement? That may help this a bit. If you have a stone foundation, then you likely always will have infiltration here. Do you keep the door at the top of the basement stairs closed? And you are on the right track as to keeping your drapes closed to reduce solar gain. Also be sure to run the exhaust fan while cooking and bathing (and then turn off when finished) As to your drafty doors, that is good to review and will help with your winter bills too.

What style and era is your house? If there are similar houses in the area, I'd try to find antique ones before spending $1000+ (with shipping and tax) per door. Check out local architectural salvage stores, Craigslist, and even eBay (you can restrict your search to local sellers). Avoid painted doors if you can because they're expensive to strip (and tedious and messy to DIY), the paint may contain lead, and you won't be able judge the condition of the door very well. Also, pay attention to whether the doors open from the left or right. Be sure to bring pictures and measurements (width, height AND thickness, as well as the sizes of the panels and the width of the stiles and rails) when you go to look at doors, especially if you're trying for an exact match. There are lots of subtle differences between doors that you might not notice if you don't have good pictures for reference. For example, I'd like to replace a few of the 4-panel (2 over 2) doors in my 1870's house, but so far, I haven't found any with similar proportions (long upper panels and short lower panels) and panels. (My panels are raised, but only slightly.) I was amazed at the variety of 4-panel doors out there. (I decided to stop looking for now until I finish some other projects.) On Craigslist, search under Materials. Lots of contractors sell materials that they remove from remodel jobs, usually more cheaply than you could find them at salvage stores. You can also post a free wanted ad, but be sure to have great photos and be very specific about what you're looking for.

@cpartist: Thanks for the image. We've previously discussed doing something similar but with an attached bathroom to effectively create a garage apartment. Unfortunately, the windows shown on the back wall are actually only skylights. If we close off the double-mulled window at the left side of the existing playroom to create a bedroom, that leaves 1 skylight for the playroom and hallway. Even with adding another skylight the areas will be dark. Also not sure about creating that bedroom without direct access to a bathroom, which forces guest/children to walk through a playroom to utilize it. The more we look at this and consider everyone's comments, the more we think our hallway plan above addresses the most needs for our family. If we, or some future buyer, needs a downstairs en-suite then that could be added without difficulty. Thoughts?

What do you mean by "digging out"? If you're actually talking about digging out a dirt floor cellar ... definitely rethink that. Do you have a garage or even a carport you could enclose or an unused bedroom you might remodel instead? Do you have room on your lot to build a screened porch now that could become an enclosed sun porch later?