tear off layers or install on top?

Hello, I recently purchased an old (~90 year) home with a roof in rough shape. The current construction consists of two asphalt layers at the end of their useful life, over a layer of cedar shingles. I think there is a layer of tar paper between the asphalt and cedar. The decking consists of horizontal boards with large gaps between them... I understand that this was to help the cedar shingles breath. I'm interested in a metal roof but I don't know how to proceed. I need something insulated and weather tight. If it matters, we plan to use the attic as a livable space down the road... the roof is quite steep and the attic is large and open. I've been reading through your forum and other sites on the internet, and it sounds like there are differing opinions as to whether or not one can simply add a metal roof on top of existing materials. I was told that I might be able to just put down some 1" foam board on top of the asphalt shingles, and then install the metal on that. Is that appropriate, or should we tear off some of the old material first? If we tear off old material, how far should we go? Can we get rid of the asphalt but keep the cedar, or do we need to tear everything off and install solid decking? I won't know the condition of the cedar unless we tear off the asphalt. The roof is not leaking water (except at the damaged chimney flashing), but there isn't any insulation in the attic so I'm sure it's leaking air. The previous owners have a 1000 gallon propane tank for a ~900 ft^2 home, leading me to believe that I might go through an awful lot of fuel in the winter. What do you think is the best way for me to modernize the roof to maximize energy efficiency? Thanks for your time and thoughts!

+1 I am a big fan of removing the old roof as a standard and while not necessary in many applications, I agree with Todd here 100%. Do it right once and be done with it.