If you purchase any GPS device for serious use, make sure that it has enough memory to be able to load good maps. Generally the maps supplied with a GPS are very basic.
The other thing is that it must have a I/O port and cable so that you can load your own large scale maps, routes, waypoints and comments etc. The ideal software for managing your data is called OziExplorer. It allows you to transfer your tracks etc onto and off of third party scanned maps in a number of different formats, GeoTiFF, jpeg, bmp, you name it
Below is a copy of a 100km round trip from my home to an old goldmine and return the larger is at 1:50K scale the other at 1:1M The larger being a portion of the smaller at the bottom right hand corner of the track.
Altering routes and wayponts is possible on most GPS units, but is much easier to do on your 'puta. It may then be uploaded to the GPS.