Here is a quote from a previous post about building a lower receiver. The lower is pretty much a kit project, once you have gotten the required parts and a few tools. From what I have seen on YouTube, building the upper requires more "gunsmithing" skills and some additional tools. The quote below is from this thread,
http://www.450bushmaster.net/viewtopic.php?f=2&t=47&p=27133&hilit=lower+build+mako+gl+shock#p27133.
"Just completed building a lower for my 450. If anyone is contemplating doing this, PM me and I can send you a list of the parts you will need. Building the parts list was one of the harder aspects of doing this. I undertook this because I wanted some custom components, so I ended up with the following:
Tactical Innovations stripped lower (Tactical Innovations)
DPMS / Panther internals (Brownells) and buffer assembly kit (DPMS)
Magpul trigger guard (Midway)
Mako GL-Shock butt stock (Outdoor Oasis)
Mission First Tactical G-27 Tactical Pistol Grip (Midway)
Geissele two-stage trigger group (Midway)
I could have saved some money by shopping more widely, but the prices at Midway and Brownells were competitive. Also, Brownells sells an internal parts kit without the trigger group or pistol grip, which I planned to buy separately. They also have a set of instructional videos at
http://www.ar15builder.com that were invaluable. The trigger group ran the price up to just south of $525 including shipping, and I also invested about $100 in tools (roll pin punches, pivot pin detent tool, AR15 combo wrench to tighten the butt stock, lower receiver vice block, etc.), since this was my first build. It took me about 6 hours to put it together, excluding time spent searching for the take-down pin detent, which launched itself into outer space during assembly. Yes, you could buy a less expensive lower with stock parts.
The upper is stock Bushmaster 450 with the addition of a Harris 9 - 13" swivel bipod, Butler Creek sling, and a Sportsman's Guide (I think Barska makes it) 3-12x50 scope on a UGT riser."