Do you mean price or design?
You can get a 35' D50 just 2 reel arms, sort of a cheapo header.
Yes. But only on 35' and 40' FD's. FD's run 2 hydraulic motors while D60D's run mechanically timed knives.
This might help: http://www.macdon.com/chart.php
Our 7010 has a 36' single knife flex draper and works great. I wish we had the extra 4 feet. I'd rather drive a little slower to keep her full and get a nice cut than race around at 6+ mph. I guess Macdon better get working on some bigger better drapers for the class 9's. But I'm sure they're already on it. My vote is for the 40 footer.