Remember that when off road you can't have auto routing, just direct lines between predefined waypoints because there are no vector mapped routes for the GPS to use to calculate. If that is all you need even my ancient Garmin 12 would give you 100% success. You just enter the grid refs of each waypoint and follow the arrow. A copy of Mapsource is useful as its easier to enter the waypoints on the PC and download them, but not essential.
If you want to go upmarket there are units which will run Memory Map but you need to budget £300+. I use a Ipaq Travel Companion which runs Tomtom software (as purchased) but now also runs Memory Map for off road, in my case walking, and Garmin software (better than Tomtom). In practice I find memory Map as a planning tool on the PC is very good but loading the waypoints of a route onto the old Garmin is actually more practical than using the Ipaq as all I need when moving is a big direction arrow. The old Garmin uses standard AA batteries that will last a day, the Ipaq uses a dedicated rechargable that will last a few hours. I also find it better to take an OS map than use the small Ipaq screen.
If you want a GPS for full public road routable navigation it may make sense to spend the money for a dual purpose system. If you only want off road point to point KISS comes into play - Keep It Simple ****