DTorrance I have been reading your post with some interest. I too live in Maryland only south of you. I own two houses, One was my grandmothers that I own now and the other one is our home. My grandmothers old house we rent out and we have had to replace three systems in our rental house. For some reason the builder who first owned the house wanted baseboard heat upstairs, a heat pump downstairs and an air handler in the attic. So when those things need replacing it can get kind of expensive. Our tenants are great people but tend to keep the heat up and a door open at times which just taxes the systems so that just makes it necessary to call the repairman in a little bit more often.
I thought I would tell you how much our boiler cost and how much the heat pump was so you can get a better handle on the cost of what you want to put in. The boiler was $8,000.00 and the heat pump was $7,000.00 to replace.
I think you are going in the right direction when you say you are thinking of going the heat pump route. The heat pump would definitely be cheaper to install and I personally think they provide enough warmth as compared to just plain old forced air heat with a furnace which is what we have here at home.
There are a few things to consider though. From what I understood of your post you have an air handler in the attic. Since you do have an air handler in the attic you might want to have your filter in the same area as your central return. That way you will never have to go up in the attic ever again. As for returns with a central return you really don't need any other returns unless you really want them. I myself if I had built the rental house would have put returns in every room and would have run the duct work from a basement furnace to the attic for the cool air/heated air and then return ducts in the basement going to all of the upstairs rooms. It might have been more expensive that way but to me anyway more efficient and it would have been at the maximum only two systems instead of the three it has now.
Another thing too that another poster mentioned already,insulate. The one thing I can't complain about in our rental house is that unlike our own house it is well insulated. At our house we have begun some insulating in our bedrooms and the heating bill certainly is better for it. Also to cut our bills down we replaced our badly warped door with a well insulated door and we have done window replacements in every part of our house except an addition we don't use that often. So all of that helps and as your budget allows I would certainly do as much insulating and replacing of doors and windows as needed.
You mentioned as I remember going with a water heater that would be heated by the boiler water. Since you are going the heat pump route you probably have abandoned that idea and I am glad of that as our plumber said they don't work that well. Instead if you do want to replace your hot water heater I suggest a tankless model that will heat your water as you need it. They are more expensive though so wait until the old water heater dies. We are thinking of doing that ourselves to save on gas costs so if anyone has any comments on those please let me know. Anyway good luck!