If 600-800 yards is your objective, you could easily get away with a PARTIALLY custom rifle. No need for a "ground up" precision build. I have a Model 70 Win in 30-06 that I can bang a 16" gong at 800 yards with alarming consistency. Only thing I've done to that rifle over the years is put in Timney Trigger and bed the action and float the barrel. I have several "ground up" builds that that old Model 70 will beat the pants off of. Then, like RR, I have a few custom jobs that will split a frogs hair at 600 yards and go looking for the frog! As Jeep put it, it's pretty much up to your wallet and wants, not necessarily "needs". From what you have said, you could more than likely do pretty much what I did to the old Mod 70 with good used rifle thats in good shape. Maybe rebarrel it with a Lija or a Shilen barrel. Or you could find a good action and have a nice custom ground up build. All in what you really just "want". But I will clue you in on a secret. The stock fitting the shooter is just as important as the barrel fitting the action! If a stock doesn't fit you well, then you won't shoot that rifle worth a tinkers damn 99.999% of the time.

As to your "narrowed down" to 6.5 Creed, at distances past 500 yards I would be a bit wary of that particular cartridge. You can only fit in so much bullet length (for weight) before you start to have to compromise space for powder and sacrificing MV. Ya need to have some down range weight for consistent penetration. Or some really serious MV and a high BC bullet to carry that speed and energy downrange. The Creed is fantastic for punching paper at 1000 yards but past 500 I just don't think I could trust the penetration capabilities of that round.