Yup, gonna try my luck at my first enduro next Sunday. I was hoping you could've made it so you could ride down with us.
The update shouldnt matter, Garmin gives you free (not maps) unit updates you can download from their site. If there's a new map version coming out, you can wait to buy it after it's released to get the latest maps since they charge you to update that.
The 450 & 550 mounts are the same, it's just that the 550 also comes with a car mount. The bike mount holds up just fine for off-road, city potholes/curbs, and rain. No problems so far. It's designed to be pretty sturdy. There are some unit failures out there but it's not a very common thing. For the most part Garmin stands behind their products pretty good so if you do encounter a problem they will make good on it.
I'm no GPS exert but you can find a ton of specific info on the Zumo here... http://www.zumoforums.com/
No free weekends till June? That means you cant make the Mass dualsport ride in May either