New roof? Remove all 3 layers of composition shingles first?

I want to remove all the shingles and have new decking applied before putting on a metal roof. A friend told me to just apply the metal right over the shingles. Won't this be to heavy? Also since there are 3 layers of shingles on one side of the house and only one layer of shingles on the other side of the house wouldn't it be to uneven to just apply the metal without removing all the shingles? My house is old and there is no solid decking up there just 1 x 3 's running across the rafters and shingles applied to the 1 x 3's. Wouldn't solid decking be more stable? And wouldn't solid decking hold the metal better? Help me out here please I don't want to over spend if I don't have too but I don't want a shoddy job done either! This house is where I will spend the rest of my life so it's for me not for resale value! Thanks for any help you can offer, I appreciate your time and care you take to answer my questions!!!