You mention it being a very large room, so I'd plan on external amplification, unless your volume requirements are moderate. That's easy, though. The toughest part is the speakers, because no one can really help you a lot - they are so subjective, that what one person would recommend highly, another would warn you away from, simply because of personal preferences. The room itself will play a part in what sounds good. Does it have lots of hard, reflective surfaces, or is it carpeted with soft furniture that absorb sound? My own opinion is that if the speakers are right, pretty much any brand of good quality electronics will work, and the differences will be relatively minor. Denon, Marantz, Pioneer, Yamaha, Onkyo, etc, all make good stuff - each with their proponents and detractors, but no major differences in sound quality and power delivery.
This can be tedious process, but the results of taking the time to audition carefully, and buy even more carefully will make the difference between a $15K system that sounds like $5K or $50K.