Did you allow an overhang on your roof? 18''-2' is what i would recommend with slotted or bb'd soffit. If i was gonna have a lift in there I agree 56Fordguy on having a minimum of 12' walls. Lay 4' worth of block off your pad and use 8' boards is what i would do. Use 2 salt treated sil plates( 2"x8's") bolted onto the block. You'll have bolts concreted into the block, drill holes in your sil plate and use a washer and nut to connect the sil to the block. I'd do it every 4' or so- I really don't know what the code is or even if there is one. I'd use 2 2"x4's non treated for my top plate. I maybe wrong, but i believe the wrap don't really insulate it just keeps out the wind and moisture. I've never really messed with metal framing much, so personally I would forget about the whole metal truss thing and just stick build it. At least 4''of concrete sounds about right, and no matter what it will crack (never met a concrete guy that guaranteed it not to crack). Is your building spot pretty firm? You gonna have 2' footers under that pad? Bigger doors the better IMO. I'd be happy with a 12' carport...If all my toys would fit under it. I'd slat the roof on 2' centers with .5"x 2"-3" boards from a sawmill, and use metal roofing. I'd use roll insulation for my walls and spray in the ceiling.