Summarizing the experience reported here, I'd start with the mapping handheld units listed at https://buy.garmin.com/shop/shop.do?cID=145
. (I know the Garmin line; other manufacturers can compete, no doubt, but Garmin is the dominant market leader.) They will all do the job nicely. Among all those models, consider these first: Etrex Legend HCx, GPSMAP 62, and Oregon 450.
For readers on a budget, the GPSMAP 60Cx is worth a look. Discontinued by Garmin, but just as capable as the current units, it is widely available in stores at a good discount.
Move up in each line if the extra features appeal. All GPS receivers provide altitude; the barometic altimeter is more precise. The electronic compass is useless for cycling, IMO, and draws a lot of battery power. The larger screens are nice, but also reduce battery time.
The first map to buy is City Navigator. Add Topo 100K for contour lines, some trails, rivers, and lakes. Get them on DVD, not the preloaded MicroSD cards, so you can use them on your computer to set up maps, waypoints, and routes.
Get a handlebar mount, of course. Plan on a backup attachment--lanyard, rubber band, etc.--as the lightweight bar mounts are not very secure on big bumps and the secure mounts weigh a bit over a pound.
Four high-quality rechargeable AA batteries and a small mains charger will keep you going for five or six days between power outlets. My favorite AA cells are the Ansmann 2850 mAh.
For tips on using the ACA route data, see the sticky post at the top of this forum group and also http://www.adventurecycling.org/routes/gps.cfm
, where you will find a link to all the details and step-by-step instructions in the GPS Data User Guide.
Finally, we have discussed all these topics here in much detail. Try searching from the page that lists the GPS Discussion topics, using key words from these summaries.