Mark,
I didn't do the Nav unit. And really don't plan on it unless would just stumble in to smoking deal on eBay or something someday.
The nav unit is really just a navigation unit. Not a mapping unit at all. It's (basically) turn by turn local routing and a trip computer all done in the small radio display. No mapping display at all though. Got a chance to look at one installed on a bike for a bit one afternoon and really wasn't impressed. Especially when you find out it costs more than does the radio.
Like you I've got the Garmin 2620. Used a simple quickly removable handlebard mount. Also took their accessory power cable, built it a length that just effectively reaches the lighter plug with a good plug from Radio Shack, and it works great. It's not cumbersome. The cord is out of the way. The unit is either on or off the bike in literally 90 seconds.
The mapping display on the 2620 is good enough, and so intuitive, that I don't even mess with hooking up the audio. It's not necessary (at least for me), means I don't have a cord draped across the fairing or tank, and it just works great without it. Have used it on three long-ish trips and a few short ones this way and it's a completely effective solution.
As for selling the old radio; I just put it on eBay and started the auction at a dollar (that's how I always do it there). So the market set the price, not me. In fact it "sold" twice. First buyer claimed poverty after the fact; so I relisted it. Exactly the same price the second time from a different bunch of bidders. So something close to that price ought to be reachable again, at least for awhile.