you need a unit capable of taking 10,000 points in a track because some organisers send you a GPX track and a 600k, which you might one day ride, can take 10,000 points. The Map 64 from Garmin does.
You might prefer replaceable batteries in case they run out far from home.
The best first step is to find a mate with one and get them to show you how they work. it's much faster than trying to teach yourself.