You've posed a "how long is a piece of string question".
What's your budget?
You've probably got three basic level of sophistication here:
A basic unit that will guide you to a position; you'll need to carry maps and printouts of the cache details (or carry a PDA with the appropriate software)
A slightly flasher unit that has a mapping facility; you'll still need to carry the cache details
A paper-less unit which as well as having superior mapping will also load all the cache details on the unit
I'm quite happy with my Garmin Venture HC ( a second level machine). I got it from the States through Amazon. For caching locally I use printouts. If I'm away on holiday with heaps of possible caches I have the GPX files loaded on my PDA.
Which raises another point. If you are going to get serious with this game you will need to get Premium membership. This allows you to build sophisticated queries of cache location; place; age; type; etc and download them as a single GPX file. You can then upload them into your GPSr. As a non-Premium member you can only upload one cache at a time.
Hope that helps.
Regards for that .
i am looking at a garmin oregon550 which does all the stage 3 things you listed, anyone out there have one or have found any faults etc,