convertintg old stable

i've read a few books (jeff cooper's building a recording studio, everest's small budget recording studio/12 designs, and ordered rod's) in addition i've read through a bunch of things here. still i find myself confused about how to procede. here it is. see sketchup file. a very old stable house: 23'x17' ceiling slopes from 10'8"-7'6" from the front face (23') to back. framing is all 2x4, 24" on center. siding is very worn appox 6"x1". roof is 12"x1" with plywood and shingles on top. there is a huge wood beam for ceiling support running across all 23'. structure is freestanding with a slab that at least 12" - 32" think. that's what i can measure above grade. it's on a hill. windows and doors all need reframing. has 3 windows 1 door on front face, 1 garage door on rear face. for phase one i just want to seal the structure up. assuming the roof and slab are good. my contractor says it will be easier and cheaper to use plywood siding 8'x4' over the existing siding. my thinking is this would be fine. thats one more layer of mass, a layer that will not have the same res freq of drywall or plywood. i guess it just comes down to sealing it well. and i wonder what his plan is for that since it will be sticking out 3/4" from the current foundation etc. the other idea i have is to seal the garage door with frame in place that can be removed when we sell the house in maybe 5+ years. hopefully this will lead to an easier conversion back to garage and where i live people love parking. still it should be a structurally sound wall that is sealed. the large pic window will be removed, wall reframed. debating the front windows. i love the idea of some light. any insight on siding over the old siding?