Hey folks, I'm new to this forum and this is my first question

I'm currently soliciting bids for remodeling a 30 year old lake house in north GA. We want to put a metal roof on the house. In order to be a little more educated about the roofing process, I'm wondering if it is ever acceptable to install a metal roof over the existing asphalt shingles (I would expect the roofer to lathe with 2X2 or 2X4). Is there anything else I should be aware of or ask about regarding metal roofing when soliciting bids? Are all metal roofs created equally?