I know the Garmin line best, but perhaps someone will add suggestions about DeLorme and Magellan.
The eTrex Legend HCx sounds like a good fit to me. Equipped with the City Navigator map set, it will take you through the 50 states and most of Canada. Buy the maps on DVD, not the preloaded SD card, and put them on the Mac for your trip planning and reloading the receiver with maps, routes, and waypoints. City Navigator does not include elevation data. The ACA maps show contour lines at large intervals, and their GPS waypoints include elevations in mountainous terrain. You could buy the Topo US 100K map set for full elevation coverage.
Having the Mac eliminates the need for huge map storage or waypoint storage in the GPSR. You will probably load maps every few weeks of riding. Other features of more expensive models: magnetic compass and barometric altimeter. The Legend shows your direction as soon as you start moving, but not when stopped. Its GPS altitude is less accurate than barometric, about +- 50 feet vs. +- 20 feet. Both are fine for cycling.
Use the free RoadTrip software to manage the maps, load them, take your daily tracks and waypoints, and plan routes.
Accessories you are likely to want: handlebar mount, 4 rechargeable AA batteries and an AC charger, MicroSD card (probably 2GB size), and card reader for the Mac if it does not have a slot. If you get a camera that uses AA batteries, it can share the charger and probably the card reader.
This unit does distance and speed, but not cadence nor heart rate. (The Edge series does, but it is not suited for touring, being specialized for training.) No temperature, either, but a keychain thermometer does as well.
I still use a bike computer beside the GPSR, mainly for cadence and as backup for distance. GPS eliminates the mental subtraction exercises that we all did when estimating the next turn.
GPS takes some practice to use well. Plan to spend an hour in an easy chair with receiver and owner's manual, then take it on local rides to see how it does on routes you know. A few hundred miles will give you a good idea of what it can and cannot do.
Finally, be sure to have paper maps and a keychain compass along. Electronics gets lost and broken too often to not have a backup. I think of the GPSR as an adjunct to the ACA maps. It is a good navigator, but lacks the cycling knowledge built into those maps.