That's a good choice. The Garmin is a nice little device, and the mapping (and re-mapping, for those of us who tend to wander afield) works very well. Saving a bit with the Explore is a good idea, assuming you don't need the extra training hookups. The random-route is cool; it might be additionally useful exploring your area. The Explore is just a skosh slow mapping out routes, not actually annoying, but it's good to give it a minute to make plans.
Alternatives - there's the Wahoo Elemnt. Functionally right between the 820 and the Explore, and cheaper than both. It is a little chunky, and needs to be paired with a smartphone for configuration and updates; it's also lacking the randomizer. Good at mapping, though, if you do know where you want to go. Or, the Magellan 500, which does have the random route, and a ton of features, and a pretty sassy display (easy to tamper with mid-ride). It's also possessed of a higher pricetag, but not alarmingly so. Both might be worth a look, if you're not committed to Garmin. If you are, the 820 is a great choice; head and shoulders above the 520, and 150 bucks cheaper than the 1000.
Hope this helps!