I built an upper around that barrel. I love it with my KMR. But Rainier has a compete upper as well. That would be worth a look. I think it's their RUC series.
There is no reason you can't have a FSB and still have a free float handguard... I have two with that setup myself:
http://i281.photobucket.com/albums/k...ps70a6768b.jpg
http://i281.photobucket.com/albums/k...psd9583458.jpg
I have no problem with a free float. I just don't see the point, given the extra expense and weight. I don't plan to put optics on it anyway, so the increase in potential accuracy won't be very great. In the Corps I shot out to 500 yards with an M16A2 on the KD course, and I don't really need more than 4 MOA realistically anyway. But if a free float could be had without much extra expense or weight, then sure I'm game.
The more I read, the more it sounds like grabbing a basic 20" upper and chopping the barrel sounds like the best option. Every 18" barrel I've seen is built specifically for a level of accuracy you're not looking for, and mostly at prices that wouldn't be worth it for your needs. It would likely be cheaper/easier all said and done than having a custom upper built and FSB pinned. Any required change to the gas port could easily be addressed while the barrel is being modified.
See, that's a misnomer. Do you know how heavy the FSB is? MOE handguards are also heavier than you think.
My gun is 7.5 lbs unloaded, with T1 in an LT660, inforce wml, mbus sights, precision armament EFAB, bad lever, and iwc keymod sling mount.
This is with some V7 parts and the 13" KMR. BCM stock.
You can't get a FSB 18" with this barrel to be that light, or to have a center of gravity that close to the shooter.