For less than £100 now, you cannot go wrong with a Quest. Mounts that allow you to power the device from the bike are trickier to pick up, but do become available. A surprising amount of guys and gals on here still use the Quest and its uncomplicated and easily understood. Mapsource is the essential PC program you will use with the Quest to plan trips/routes etc. Its available as a download. The ergonomics of the Quest are nowhere as good as the Zumo's but "you pays your money..." etc etc. It makes up for it in other ways.
Loads of help on here either way.......dont be shy about asking.
Be careful about the Nuvi range.........some are now quite good, but mostly intended for A-B driving, rather than the use you'll likely want from a GPS.
Thats an OK price but just to put you wide, if you buy from ebay make sure its from a registered Garmin seller and its not won by auction.
If its either of above Garmin MAY not cover the warranty and WILL not supply you with your first FREE update which you are entitled to.
I found that out the hard way.
Having said all that, if you want to buy new at a good price and forego the first update, I had no probs with Pentagon GPS on ebay.
HERE is a zumo 500 for less, I don't think there is too much difference between the 500 550, and this is a good price. A mate of mine got one from them and it is a "white box" unit with a full warranty...