I actually think a Garmin 500 (or 520 if buying new) works quite well for rando navigation where you are only trying to follow a predefined course.
It tells you where the next 4 or 5 turns are, in order, in terms of distance to your current location, and it lists in text what the next turns are called (i.e. the road name). And it beeps if you go off course, and keeps yelling at you till you get back on course.
(Although this doesn't help you if you decide to ignore it like I did on a Fleche a few years ago (long story)).
The main thing it doesn't do is navigation on the fly. This would only very rarely be needed or useful on rando.
There is no mapping to speak of, of course.
But it works great as a cue sheet supplement / extra navigational insurance.
And battery life is well addressed by taking an external battery and small charger for overnights (like on a 600K).