Believe the owner's manual, a free download from the Garmin site. I have found Garmin salespeople to be consistently uninformed. (A small sample of them, I do admit.)
You are right, William; the Edge series are excellent training devices, essentially cycle computers on steroids, but they are not very good at long routes in new terrain. For that, the eTrex, GPSMAP, and "western states" series excel.
The Southern Tier GPS files total 2264 waypoints. Even the budget eTrex line will do it with a couple of reloads en route, as described in some posts here. With some work at home, Mike got TA plus WE into his Oregon 450 by using route points, which are a limited form of waypoint, used for navigation only: not editable and no text descriptions.
You are right again that getting used to the device is important. They have so many features and so many options that a couple of hours in an easy chair with the unit and its manual are well spent. Then use it on some local rides to learn its quirks and limitations before setting out.