You can get the upper assembled from
http://www.bravocompanyusa.com/ for a reasonable price and get top quality. You can also get a quad rail fore arm but you will have to do a bit of measuring to figure out how you want to go. There is no limit on how much you can spend but $100 should get you a good quality assembled upper.
The pinned flash hider may definitely be a problem. Talk to the folks at the BCM 800 number and they can give you the right info. You will have to demount your front sight/gas block combo. If you want the gas block under your forearm you may want to replace the gas block with a lower profile unit. this would mean leaving your front sight off and getting a rail mounted replacement. I recommend you get a clamp on gas block, not the type that is secured by set screws. Most folks don't realize how much a set screw deforms the barrel, just my not so humble opinion.
The two other things you will need are an upper block, there are two varieties, one that encases the upper and one that the upper is pinned to using the regular mounting pins. I got the kind you pin the upper to for about $45. The other piece you will need is an armorers wrench, about $30 to 35 from DPMS. You pin your upper in a vise with the upper block and use the wrench to demount and remount your barrel. And you should have a torque wrench. Some folks just eyeball it, I like a bit more precision. If you get luck and everything lines up perfect ok, but if it does not then having a torque wrench becomes important.
You can go to here
http://www.ar15.com/forums/topic.html?b=3&f=4&t=226782 for step by step instructions to see what you are getting in to.
As for how hard is it to do, well its basically easy if you have the tools. You can assemble an AR from parts in about two fairly leisurely hours. The only trouble is that it is habit forming and it is an expensive habit. After all, when you are done you will have a spare upper and a hand guard.