Sorry, I just opened your plan. If you have to have airhandler and ducts in the attic, then you should seal the attic with foam. The only problem will be slightly lower shingle life (3-5% less), so buy a good quality shingle. Make sure contractor uses low u factor & SGHC windows and seals then with low expansion foam. Make sure there is a room by room heat gain/loss analysis done. Make sure contractor seals all holes for wires, pipes, ducts, chimneys, ect. Mechanically ventilate. Good luck.
You know what, i applaud you efforts to find the best comfort solution for you home. Most people underestimate the importance of thier climate systems untill after it's too late.
It still amazes me how people are so concerned about what color their lamp shades are going to be, but could care less about making decisions on their heating.cooling systems.
I happen to work for a company that takes care of the mechanical needs for extreamly high end homes, and a certain TV remodeling show everyone watches. Most of the nice homes I do are insulated with open celled foam. Iconyne. My attic systems are always placed in an un-obtrusive area, and a wall is built from the roof down to the floor to form a "mech room" around my unit. The walls of this "room" are also iconyned. This is expecially done because 90% of my units that supply heating are hydo units. Again....high end homes, they're almost all radiant floor heated. So, my units are secondary backup heat for areas such as great rooms and media halls within the home.
I say, iconyne your home and enjoy the comfort !!!