First, is it even legal? Second, is there anyone out there (besides USMC gunsmiths) that can build this weapon? If so, what do you think this would cost? I know it all starts with the remington 700 action but from there i have the slightest idea.

Be prepared to spend lots and lots of money. Is your current rifle holing you back, or do you just want it for cool factor?

I don't see why it would be illegal (yet at least.) I don't think it could really be called a M40A3 unless it was made (customized whatever) by military armorers, but you could certainly build or have an equivalent built given the proper budget. You're talking about $500 just for the stock alone. You could easily spend that on a good scope as well. The gun itself will easily cost that much. Then figure on a match grade barrel, and having a gunsmith fit it.

Yeah it can be done, and it would be sweet. If there were ranges in S. Florida that went more than 100 yards I guess I'd want a big benchrest gun like that alot more.

Be prepared to spend lots and lots of money. Is your current rifle holing you back, or do you just want it for cool factor?

havent shot in a long time, since i was a kid honestly...the question comes from me watching way to much military channel and top sniper...haha...but to be honest theres no beter way to become an excellent shot than with having one of the best rifles in the world right?!?!? of course thats just a starting point, having a good rife isnt gonna make you the best shot...breathing, lowering heart rate, patience, range esimation....ok there goes the top sniper stuff again haha

You can get close to building one. No legal problems if you can own a firearm. Like you said start with a Rem 700. You can get the real deal stocks here http://www.mcmfamily.com/mcmillan-stocks-tactical-stock-list.php#m3a. (http://www.mcmfamily.com/mcmillan-stocks-tactical-stock-list.php#m3a)
for the barrel you can find a few places like this http://www.accuracysystemsinc.com/cust_remington_700.php

I Thought about doing it at one time, but do to cost did a nice 77/22 build similar to a M40 A1.

Here's a good article for you http://www.snipercentral.com/forums/viewtopic.php?t=11137