Hi, High.
Welcome.
IF you need some financial wiggle room, I'd suggest just going 5.1 for now. You might not benefit significantly from the jump to 7.1 and it's a pain to wire all those surrounds in a rental. You can always add them later.
But, yeah, if you can afford the cost and installation for all that, I'd do it. I actually like choosing the EP500; it will fit your space more easily than the EP600 and be easier to move later. My sense is that you don't really sacrifice very much performance.
I wouldn't worry about the Onkyo. If you run into volume issues later (which I doubt) you can always add an external amp.
