Turn by turn is only available on the 800 (or previous 705)
Accuracy is a given.
Can't download cue sheets to any of them. You can download existing GPX or TCX routes and courses or create your own on various websites. These are files which will show up as tracks on the display that you can follow and which will give turn by turn beeps and arrows on the display.
With a 500, you can download TCX files which will show up as a track you can follow on the display but there is no map and no turn by turn guidance.