The problem with our old furnance was alot of the heat went up in the attic through the exhaust pipe and we had condensation up there that rotted the roof trusses.
The reason for the condensation was the extreme temperature change of the warm/hot exhaust pipe where colder air surrounded it.
When you seal the attic (underside of roof) with foam, the attic will stay warmer (same as the living space temp) and that extreme temperature change will not occur, thus your condensation should be no problem.
The older homes with fiberglass insulation were built so that they could "breathe" because of moisture being trapped within the fiberglass.
That is not the case with foam insulation.
I would definitely install an outside air intake vent to supply the furnace with combustion air. Just enough to make up for the exhaust that escapes. In a tightly sealed house, it's a must.
I would also install an outside air (one that you can open and close) intake for the return air intake on the furnace & A/C.