If you had the any of the 26xx, 27xx streetpilots in the past, it should just mount and plug in to whatever you had...
I just bought the 2820, and the Garmin mount plate and power lead that I was sold with it, both have 2610 on the box...
With it I was sold a RAM mounting system that it bolts onto with a clamp for my bars on the bottom.
No need for an antenna on a motorcycle, the built in one works fine.
If you have an intercom, then you'll have the sterio lead that you need... you may need a "ground loop Isolator" (if you have an Autocom certainely.. Autocom will sell you one) If no intercom, then you'll need to buy speakers/earphones for inside your helmet...
I'm trying to get mine to talk to a Siemens S55 bluetooth phone with no luck right now... not a huge body of experience to research yet either...
Other places to spend money are possible bluetooth helmet, a few pounds for a decent speed camera database, lockable mount so that you don't need to remove it every stop....
Al...