There are a lot of sites that sell parts, but the thread on arfcom for the latest deals sure helped. It's in the Build It Yourself forum. Note the stickies at the top on how to assemble things. It doesn't take a bunch of special tools, some of us just used what we already had in the garage.
The fixed carry handle sight is problematic - you basically use it that way, and forget about mounting a scope at the proper height. It can't. Otherwise, most AR15 builders use the A4 flattop with rail, as it's far more versatile. You can mount the M4 carry handle sight for the retro look, or use flip ups, and then add a red dot, too.
Otherwise, it will still shoot, but you won't be able to use any optic well. Other than that, it's basically time consuming research to discover what are the best deals out there. Assembling the gun is about 5% of the effort.
Research and read, it's difficult to say what you prefer until you see what's available. Then pick your parts and find the best vendors.