I'm gonna weigh in on this one a little bit:
I love IGH drivetrains. I do. But, word on the street is, they kinda suffer with offroad use. Anything but the direct drive gear (which is "2" on a Strumey 3 speed) is a bit weak in high-torque situations. Like you might experience off-road. Also, generally speaking, you shouldn't run anything lighter than a 2:1 ratio (Based on the direct drive, meaning your front sprocket should have at least twice as many teeth as your rear cog.) with them. That's the manufacturer's recommendations; i know millions of ppl have been running lower ratios with "no problems", but if you do run a ratio lower than a 2:1 on the direct drive, and it blows up, that's no one's fault but your own. FWIW, I wouldn't want to run a 3 speed offroad with the 2:1 sprocket cog, b/c that might be kinda steep on the hills. It'll prolly be good enough for the trails near me, but serious hillfolk might want something lower.
I'm not speaking from direct experience here; I had intended to build up a cruiser frame with a Sturmey xrd3 for use on trails, but my internet research and warnings from my friends dissuaded me. Sturmey's aren't cheap. And, while they are easy and reliable to work on, they are admittedly more work to rebuild then a derailer system. More steps for tire changes, too.
To solve the brake issue, look into getting one of these (Yeah, i know the OP already built his bike up, but this is cool):
http://www.ebay.com/itm/400152111079?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649 I grabbed one; haven't used it yet, but I got it in the bin, just in case.... (126mm O.L.D.)
Came in brand-new condition, but the grease is so old, the bearings feel stiff. I'll need to rebuild before i use it.
Either way, I'm happy we got choices between derailer gears and IGH gears. Viva la difference!
-rob
Make your chain a few links longer so the derailer has a little less tension on it. 
