You likely want something with a longer barrel. It's just my opinion, but you probably want a G3 SG-1, SVD, or an SR-25. The best thing for you to do would probably be to either get a Chinese brand gun or a second-hand gun and completely rebuild the internals.
Here is an example of something that you might like:
Take an Echo-1 G3 SG1 (all you are going to use are the externals, maybe the hopup unit): $170
http://www.airsoftgi.com/product_info.php?cPath=139_24_163&products_id=3145
Next, you'll want to replace pretty much everything in the gearbox, and a Systema drop-in is a quick-and-dirty way to deal with that: $200
http://www.airsoftatlanta.com/product_p/66510.htm <--I'm not 100% that this one would fit the G3 SG-1, but I'm pretty sure that it uses the V2 gearbox. PM Dominum about it, as he has worked with several G3s (both of mine, actually)
You need a replacement motor to drive that shiny gearbox, and your 2 best options are (in my opinion):
These will give you the power to pull higher springs, giving you better range:
The Tokyo Marui EG1000 motor: (~$45)
http://www.airsoftatlanta.com/product_p/17990.htm
The Systema Magnum motor: ($95)
http://www.airsoftgi.com/product_info.php?cPath=31_50&products_id=1841
Higher FPS gives you bettter range (all things being equal...it's true that high FPS is not the ONLY thing that adds range, but a HQ setup will only benefit from higher FPS), so you could use a more powerful spring. The above gearbox gives you an M120 (or 120 m/s...putting you right around 400 fps). Your new HQ parts could pull any spring up to an M150 (putting your near 500 fps), but the harder the spring, the more power your gearbox requires. I would switch to a 9.6 V battery at the M120, and I would switch to lipo batteries at anything harder than that.
A new spring will probably cost $18 regardless of the power.
New batteries can run anywhere from $25-$50 per battery depending on the make, type, voltage, and capacity. If you have no lipos at this point, a lipo charger is a must and will cost another $30.
I would also recommend a MOSFET from SteevoLS simply because of the higher power your setup is pulling. $20
For the sake of accuracy, you HAVE to get a quality hopup! (more important than tbb, in my opinion): $18 or $7
I like firefly hard-type hopups for my high-fps setups:
http://www.airsoftatlanta.com/Airsoft_Firefly_Hop_Up_Bucking_for_AEG_Hard_p/63312.htm
Though many swear by the less expensive G&G hopups:
http://www.airsoftgi.com/product_info.php?cPath=31_170&products_id=3715
Next should be a tightbore barrel. Airsoft GI and several other places have started to make their own barrels, but I have not heard about a comparison of the quality. I would recommend a 6.03 mm diameter. 6.01 will get you some higher fps, but the risk of barrel jams is higher, and I've been hearing that the increase in accuracy is minimal, not to mention that they need to be cleaned after EVERY game, and they cost more.
ASGI barrel (I don't know if this is the right length): $30
http://www.airsoftgi.com/product_info.php?cPath=31_55&products_id=5845
Promy barrel (tried and tested quality for more $$$): $70
http://www.airsoftatlanta.com/product_p/58006.htm
Once you are finished with all these things, you can look at things like optics, externals, and maybe a foam suppressor to reduce the noise of your gun from downrange (people will still hear your gearbox, though). The sky is the limit with $$ here, but it is possible to get a basic scope for $15-40 secondhand and a suppressor for $30-60 secondhand (you'll need an adapter, which is more $$).
You can add the cost of everything I've listed, and it comes to $497 at the lower limit, and $646 if you get all the best stuff I've listed.
By the way, if you have not done work on AEGs in the past, you may benefit from reading forums and asking questions to other players about installing all your parts. Since this would involve a drop-in gearbox (gearbox work is the hardest part of AEG maintenance), you could probably do most of it yourself with some simple help. YouTube has many videos about airsoft upgrades. If you just want somebody to do it for you, Dominum will probably do it all for $35-45.
This is by no means the only way to go, there are many less expensive (and even more expensive) alternatives to this set-up, but I hope that I've gotten the point across of exactly where you should go with this. These upgrades can be done in steps, gradually increasing the quality of the gun. Usually people increase the FPS and accuracy with new batteries, tbb, hopup, and springs first, then replace the gearbox and motor when they break (which they will eventually).
An alternative is the wait for somebody who has already done all this work to fall on hard times and sell their gun:
http://www.airsoftretreat.com/classifieds/showproduct.php/product/7601/cat/2 <- M16 with internal work done
http://www.airsoftretreat.com/classifieds/showproduct.php/product/7595/cat/2 <- SR-25 with upgrades to internals, probably needs some maintenance, but most of what you need is already there
http://www.airsoftretreat.com/classifieds/showproduct.php/product/7742/cat/2 <- Echo1 SCAR with some work done, but you'll need to continue to get your high-accuracy setup,
http://www.southcarolinaairsoft.com/forums/http://www.southcarolinaairsoft.com/newforums/viewtopic.php?t=3
Please be VERY careful when buying secondhand guns, pay attention to what they are NOT saying about the gun, and ask many questions before you commit to the buy. If the seller does not answer your questions or claims ignorance, you would do well to not take the risk and move on.
