Sweet. Nice unit. I have had an Onkyo TX-SR805 for a number of years now. It has served me well. The NR515 model has more features than I can count and cost considerably less than what the 805 cost me. Seems like you picked a winner.
For the 7.1 setup, if you have a couple spare speakers around the house, try first adding those to your 5.1 setup. The 2 extras (rears) don't have to be top of the line (although a purest would argue, and I would agree with them is money is no object). I would try things out that way if possible, then decide later if you feel the need to upgrade. My 7.1 setup is a mix of different brands. A 5.1 setup is very capable, but you might like the extra dimension of a 7.1 setup especially if you have a good room for it.
I am assuming the 515 has similar audio surround modes as the 805. Many movies you might watch will only have 5.1 audio, but there are listening modes in the unit that simulate 7.1 (they take certain frequencies (or something) and route it back to the rear speakers). So you can get some of the 7.1 effect without actually owning a pure 7.1 soundtrack. I would say the majority of BR movies out there are 5.1 soundtracks, but there are some with 7.1 soundtracks (e.g., War Zone: Punisher).
I have an Oppo BR player. Slightly more expensive than other players that can be bought almost anywhere today, but a good quality brand if you are willing to pay. But I am sure there are vary capable players out there for not much expense today. These types of electronics always come down in price. Do you have a PS3 perhaps? They play BR quite nicely.
Another option, which is how I view all my BR and DVD anymore is the Netgear NEO550. Takes a little more work though to setup and use than a direct BR player. I burn my videos to a hardrive and direct connect the hardrive to the NEO550 via USB, and then the NEO550 to the Onkyo AVR. I have a collection of movies/concerts on hardrives (and an MP3 collection). If you consider an option like this, note that Netgear is no longer supporting the NEO550. There are other brands out there also. I just mention this as a playback option.
Have fun.