Your system sounds similar to mine. I have Klipsch Reference 5.1 speakers, so congrats on your gift. My AR100U is 14 feet from the 119" white 1.1 gain screen. The projector is also aligned with the top of the screen, using the Peerless PRG-UNV mount, column and ceiling plate. If brightness is what you're looking for you won't be disappointed, even in eco mode it puts out a bright colorful picture. I just got done installing a AR100U in my niece and husband's new home theater, 120" screen from 16 feet. Klipsch RF52II 5.1 speakers. Denon 2312 AVR with Panasonic 221 Blu-ray player and Time Warner HD cable box. It's located in their basement but it's actually more like a second level since the east wall is lined with floor to ceiling windows. It's a huge area with a full kitchen, full bathroom and a bedroom for guests. There's also a pool and foosball table. They had the official open house/birthday party last Saturday and everyone was impressed with the AR100U picture.
The AR100U is a 2D, not 3D projector, so if that's important you may want to look at the new Epsons.
I should have mentioned that the birthday party took place at night with all of the lights on. The majority were recessed canned ceiling lights and other indirect lighting so that the many people in attendance weren't forced to walk around in semi-darkness in the open space home theater. In spite of the abundant ambient light the picture on the large screen was hardly affected, both a testament to correctly done room lighting and the brightness of the AR100U.
All I Really Need to Know I Learned in Movies