Here are the answers to the questions:
1) I put 1/2" thick weather stripping stuck to the PVC where ever it would touch the fenders or hood - thereby avoiding scratches. It will likely still leave a "buff" mark after a season of use.
2) Total cost: PVC $65 - (basically (7) 10 footers and all of the elbows, everything 1 and 1/2" pipe except the door which is 1"). Garage door opener - $15 on sale at Home Depot. Plexiglass or whatever the material - $20 off of Craigslist, Strobe off of ebay (used) $30 and pipe and flange mount was about $8. Screws - $3.50. Clear plastic - $~25 from Home Depot (enough to do it twice or maybe 3 times), switch and wire for strobe - just laying in the barn, plywood and 3/4" boards - laying in basement, Weather stripping for between plastic and PVC - 3 rolls $12, Velcro - $8 so......the grand total was about $175. The metal roof which I will put over the wood is laying under a foot of snow down by my pole barn (leftover from the barn construction). I would count on $200 and you should be safe.
3) The Oliver Cab is not mine - it was someone elses on Craigslist - I walked on that deal and am glad I did. This cab is light and has no effects on the machine that I can see (relative to center of mass, safety or other).
4) Yep, the velcro failed at 10 degrees F - the sticky back pealed off the PVC. Gorrilla tape survived at this temperature. Should use snaps - might still do that if the tape fails.
5) Note - I intentionally cut the plastic beyond the door frame to leave a flap for a halfway decent seal.
6) I used an old foam mattress for the pieces behind the pedals next to the hood. Two pieces about 8" by 15" by 6" thick. Blocks the wind but does not interfere with pedals
7) The PVC did not get brittle - I tried running over a piece at 10F in the drive and it collapsed a little but did not break.
8) You can bend the PVC without burning its surface - I was just in too much of a hurry and using my salamander style heater. Stay away from the fumes and have adequate ventilation. After bending to shape I took the pipe outside of the garage where it immediately hardened.