You have received some good replies so far. Some of my response is going to go against some of this good advice, so take it with a grain of salt. You also need to know that my education in long range shooting and rifle building was obtained from the International University of Interwebs and Youtube. I have built 1 rifle, and I haven't shot it beyond 713 yards. That being said, here goes...
1. Definitions. Aeon and others have defined long range, short, and mid-range. Extreme long range is defined on some sites as "beyond 1000 yards". Then, there's long range shooting vs. long range hunting. You said that you are interested in hunting out to 500-600 yards. Let's stretch that to 700 yards, just for fun.
2. A few weeks ago, my friend and I were in the desert shooting our rifles out to 335 yards. I with my custom build (Howa 1500 rebarreled in 260 Rem, which is very similar to the 6.5 Creedmore) and he with his Remington 700 police (don't know the exact model) in .308. He knocked over the steel target, so we started looking for other things to shoot. I said, "hey, you see that rock way up there on the hill?" My rangefinder won't reach that far, so I spotted while he walked his shots onto it. Within a few rounds, he was nailing that rock dead center with every shot. He doesn't even have mil dots to guide him- he just did some Kentucky windage holdover magic. Later, I got on Google Earth and found that rock, and it was 713 yards. I'm estimating that the rock was 24-30" in diameter, so while that's nothing spectacular, it was fun, and taught us a lot. My point is this: a stock .308, unbedded, nothing custom, that can deliver patterns consistently under 1 MOA is plenty accurate enough for big game hunting out to 700 yards.
3. When building a rifle, it's super easy to lose sight of reality. Long ago, NBK advised me "just buy a Tikka and be done with it!" After having sunk nearly $2500 into a custom build, that advice haunts me! Easily sub-moa guns, can be had for $600-750, throw some glass, a bipod, and a few other accessories, and you could be shooting deer at 700 yards for $1100.
4. Cartridge selection: you're looking at the 6.5 CM, which is very similar to 260 Rem. I may be mistaken, but ammo (or brass) is more readily available for the 260 Rem. And if you can't find 260 brass, you can easily form it from .308 cases. That's how I started, until a friend gave me over 200 spent 260 cases! 8)
5. Here is an article that will give you an idea of a true "budget" build can look like:
http://www.longrangehunting.com/articles/long-range-rig.php
There are many opinions given in this article that will contradict some of the good advice you have received, while bolstering other advice. For example, the Savage/Stevens action. The prices in the article are now dated (I don't think you'll find a stripped Stevens for $150 these days), bit I did recently see a Savage on Gunbroker sell for $215. I don't think he mentioned it, but many discussions of budget builds include the SWFA Super Sniper scope:
http://swfa.com/SWFA-SS-10x42-Tactical-30mm-Riflescope-P499.aspx. For $300, if it lives up to its reputation, it's a good deal.
6. So let's talk about a custom, budget build. As stated in the aforementioned article, you need binoculars for long range hunting. Let's assume you already have that. You also need a rangefinder that will CONSISITENTLY, every time, give you the precise distance to your target at the ranges you intend to hunt. You probably already know that you will get that kind of performance at less than half of the advertised range listed by the manufacturer. If it says it can range reflective targets at 1000, you might get consistent ranges on real targets at 4-500. Right now, I can't truly hunt or shoot at long range, because my rangefinder is really only good out to 350 yards (and it's a $350 Leupold!). Here's a basic build, not including accessories (case, sling, scope caps) and other incidentals or paint:
Donor Savage - $250
Criterion barrel from Northland Shooters Supply (NSS) - $300
Rifle Basix trigger from NSS - $85
Boyd's Stock - $100
Harris Bipod with tilt, locking handle - $150
Vortex Viper HS-T - $550
Vortex rings - $30
Vortex bubble level - $30
Nightforce or Talley 20 MOA scope base - $60
Tools - $100+
Total (not including tax, shipping, DROS, etc.) $1655
Alternatively, you could order a Brux barrel for $285 and have it chambered and installed by a good gunsmith for $275. You wouldn't need the aforementioned tools, so that brings you to $1815.
I would also recommend a compensator. Those vary in price, but let's say you spend $100 on that, I think you could get it mounted for $50, bringing you to $1965.
