Not separates as traditionally thought of, just an external amplifier for a few of the channels. I am no expert, if you want to go that route @JohnRice would be an excellent resource.
Yep. Have sold 2 friends on getting just this model recently, it was a step up from them. Watch Frys for great pricing, and ask BestBuy if they will match it. They don't technically have to but my friends didn't have to do any convincing, BB jumped at the chance to do so.
If you are good with 7.1.2 Denon 3500. If you want/need to go 7.1.4 (which is niiiice) then Denon 6500. Others here will try to sell you on using an outboard amplifier instead for the extra 2 channels but that is my my advice.