Some on this site might disagree, but for $1000 retail, you can meet your needs. It wouldn't be optimum, but would definitely meet the requirements you listed :
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-Capable of shooting accuratly out to 600-800 yards
-Capable of taking elk at up to these distances
-The flatter shooting the better
-Effective cost of shooting
-Possibility of doing a custom build from this
-Ease/cost effectiveness of improvements
-Portability, durability and most of all, reliablity
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What I would do. I'm biased to the 7mm Rem Mag, so thats what I'd choose for a caliber. I'm also biased to the Remington 700. The ADL is the cheapest version of the 700, but I've seen these shoot 'lights out'. Here is a Rem 700 ADL in 7mm Rem Mag on Gunbroker for a buy it now price of $370
I'd then buy the absolute best optics I could afford with the remainder. Considering that you will need $50 or so for some reasonable base/rings (Leupold base/rings will work fine).
Leupold base $21
Leupold 1" Rings $23
Then you need a scope, we have around $580 left for a scope.
Here is a Leupold 4.5x14x40 AO Matte for a buy it now price of $520.
This leaves you with $60 for shipping/handling etc. Now if you can save some money through your employer, thats even better.
You'll probably want to bed the action, that will cost you $1 for a roll of electrical tape, $5 for some JB Weld. And a couple more dollars for a release agent.
If you reload, you still have enough left over for dies and a shellholder.
There you go, a legitimate 600-800 yard rifle for $1000. You will also have a rifle that can be upgraded in the future (stock, barrel etc.). You can use the duplex crosshair on the Leupold scope as a rangefinder out to 600yds given some practice (a laser rangefinder is MUCH better though). I've personally used a reticle to range and take big game to nearly 900yards, but it is NOT quick and takes a lot of practice.
Hope this helped,