i'll take a shot at your first question. generally, codes say you
still need ventilation below the sheathing. this is because if
moisture finds it way into the rafter cavity, it can condense on the
underside of the sheathing. ideally, a vaulted ceiling (at least in
minnesota where i live) would be deep enough for a 2" airspace and 12"
of insulation. the venting requirements are based on the square
footage of floor area. i don't remember them exactly, but someone who
knows my well post here.

however, another option might be a hot roof---that is spray foam
(icynene) with no airspace. you will need to talk to your building
department to see if they will let you do this...we have had some
battles about this issue with our building department.

some people (myself included) feel like venting roofs is a waste of
time and money in terms of heat flowing out of your ridge vent. if i
was building my own house in a non-inspected area, i wouldn't hesitate
to skip the venting, fill the rafter cavities with insulation, and
spend my time on an excellent vapor barrier. after all, we don't
ventilate walls. building codes don't agree with this view, so we are
still venting where i live.

as to your second question, i'm somewhat out of the loop on these
prices. and they are going to very widely from area to area. but
suffice it to say that a standing seam metal roof is going to be much,
much more expensive than shingles. double the costmaybe? if you add to
that getting a contractor to blow in icynene, you are looking at a
pretty pricey little roof. nice if you have the money, but not cheap.

good luck.