How should I spend $1500 for speakers and an AV receiver? It needs to work with our old analog Sanyo 27" TV and budget DVD player and Comcast cable.
Must the center speaker be horizontal? Can you fit a bookshelf upright? The
best possible choice in center speaker is the one identical to the mains. You also would not suffer this unwanted effect called "lobing" with a horiz mtm, and you'd have the best acoustic match possible. The only way you could do yourself better is by having all three on the same plane.
What's nice about some of the ID (internet direct) brands is that they will much more likely will be able to sell you an individual bookshelf.
with a huge ceiling to floor window 11.5 ft long to the right of the TV. The ceiling is 8 ft.
Once, or if, you want to get serious about your acoustical space, you'll want to do something about that, particularly if that window is closer to you, and the other side is "open" so to speak. Something like a heavy curtain . . . and once you get the disease real bad like some of us have, you might even buy acoustic treatments on stands that can be placed behind the curtain for critical listening.
I know, I know, nutz, but I'm warning you, better stay away from us, cause we're still contaigous.

Sound quality, ability to use one remote, and smooth interaction with the Comcast are of concern.
If you have issues with smooth interaction with Comcast, it will be extremely likely it's the cable box's fault, and not something else you buy. That said, for remotes, while Harmony is very popular, I cannot recommend URC enough for the money, having used and programmed multiple of each now. The RF20 w/ blaster is $80 shipped.
Thanks for your ideas and suggestions for equipment. The budget should include speaker stands and some cables. One system that intrigues me is the Onkyo HT-S9100THX 7.1, but perhaps going with separate components will get me better sound?
Cabling = Monoprice. There, I just saved you hundreds.
Stands = I like Studio Tech for the money, but a lot of their stands have poor wire mgmt. You can drill holes to fish wire if you really cared, otherwise, pretty sturdy.
Speakers = an ID brand that I've been impressed with is Ascend. Cheers.