Seriously? Make a custom rifle.
Okay, I'd say base your rifle off of the Remington 700 series of actions. I'd say go with 7mm Remington Magnum for a cartridge, it's got a flat trajectory and it's a very fast round. You can find yourself a nice fluted barrel for not a whole lot, but how heavy a barrel you'll need will depend on the type of shooting you're planning on doing. Also, the length of your barrel will have a lot to do with knock down power at range, the longer the barrel the more 'power' you'll theoretically have at range.
http://www.cpcartridge.com/7mmB.htm
Next is the stock, I'd recommend a McMillan stock that supports a free floated barrel, which will help with long range accuracy and holding a zero.
http://www.mcmfamily.com/mcmillan-stocks-tactical-stock-list.php#a5
Stuff like trigger jobs and bipods are something else to consider which will make your rifle a thing a beauty. In my personal experiences you WILL need a custom trigger job for shooting out past 500 yards.
You can DuraCoat the rifle after completion to give it your own look.
http://www.lauerweaponry.com/
Finally you'll need to consider optics. I'd recommend a SWFA Super Sniper, I've heard nothing but good things about these scopes and for $300 you can't go wrong. I've talked to lots of guys who use these scopes out the the 1000 yard range and they say it handles those distances wonderfully. But Leupold, Nikon as well as John Unertl all put out quality scopes but they'll cost upwards of a grand.
http://swfa-ss.com/
Btw, word of advice, I've never seen a M82 for less then $8,500 and I live in Texas. One more thing, I see that you're pricing out ammunition for these rifles. I realize that a .50 BMG is alot more expensive, but with match grade rounds for even a .308 Winchester will still run you quite a bit of money. Not all ammunition is equal. In regards to weight, grain and consistency with each cartridge.
Personally the furthest I shoot on a regular basis (5 times a year or more) is out to 600 yards. But I have done some work out to the 1000 yard line. I don't know how much shooting experience you have, but you'll need to be pretty damn good to hit consistently out to 800 yards.