I use a Garmin Edge 305. It logs where I've been and I save the routes for the future. I also plot routes on GarminConnect using satellite view (more accurate for off road routes) using an OS map as a reference the upload the gpx files to the Garmin. I take it from your post that you haven't used one, so I'll explain. You save the route as a course, then select "follow course." A black line appears on the screen with an arrow which shows your location. As you ride the arrow follows the trail, and if you stray it bleeps and shows you which direction you need to go in order to get
back on course. If you ride the course more than once the device remembers and shows a ghost rider arrow which is you last time you rode, giving you something to compare your performance if that takes your fancy.
You can also search gpx sharing sites for other peoples' routes and download them to your device. There is a way to add way points and notes, say for complex junctions, but I'm still finding my way through this.
I'm not convinced you need any more than that, but I always take an OS printout of the route with me as backup ( and to find bailout routes if I need them)
So, as fr as I'm concerned you don't actually need expensive base mapping, so would recommend the Edge 200 or 500. You can pickup 305s if you look for them.