I actually thought about this and I did get about 15% of the way figuring out which parts and what I needed. Scrapped the project before buying anything but I can give you a hand with the parts that I selected.
Basically it would have cost me about $1000 to build. If you have that kind of money to throw around then great but as it stands if you want high quality custom/aftermarket parts then it's going to be a lot.
Also keep note that you should stick to one or two aftermarket brands, don't mix and match, each aftermarket company will have different tolerances for one another and although will all work with TM guns (the "standard" they are usually based on) other companies may not play well with each other.
As well if you're not in Engineering or something related you're probably going to have a really hard time. It's easy to follow instructions but there's also a thought process that needs to go in as well if SHTF and you need to come up with your own solution to a problem (and believe me in every project there are problems).
Also get AV'ed that's pretty much the only way you're going to find a high quality metal body. I haven't seen any for sale not in the classifieds and it WILL likely get seized if you try to import one yourself.
EDIT: Crap can't find the file I'll keep searching but basically what the parts list boiled down to was a G&P Metal Body along with a Modify FTK (I think the S110+ spring for 400 FPS) and the parts that it was missing and a gearbox shell (I think it was either HurricanE, Modify, or G&P). The Hop up unit and rubber I was going to use were Prometheus Neo Strike Chamber Hopup and Prometheus soft.
The OTHER thing I was looking at was a G&P body, SystemA REVOLUTION
Mechbox or SystemA Complete
Mechbox and various high quality externals.
EDIT2: Does anyone else hate Redwolfs new layout? It kind of hurts my eyes but I guess it's just because I'm not used to it ehobby has a white background as well but it doesn't bother me for some reason.
OP: There's also the SystemA ENERGY
Mechboxes as well. It's the newest bleeding edge technology and although I don't know if it's that much of an improvement (over the REVOLUTION or Complete Mechbox) if you have the money to drop and like being bleeding edge you'll be one of the first people rocking the SystemA ENERGY stuff.