The part hanging off the back is to pull the skis from trailer onto the dolly. The above picture shows it off though. In the first post you can see it attached. Two wing nuts is all it tskes to attach.
First off, I live in a small town in Colorado so things are a little more pricy. I believe the stuff I ordered on line costed me $120. 8 8" castors from Harbor Freight, 8 bunk supports and bunk carpet from Karavan, and the winch and winch strap from e trailer. Locally, wood $60, 10 8' 2x4's, and 2 8' 2x2's. Hardware at the local Ace Hardware, $64. So basically $250 for two. The hardware and wood will probably be cheaper for most of ya.