What these guys are forgetting to tell you is that to make your 30-06 a .300 Win. Mag, you're either going to have to get a new bolt (around $250) or have the bolt face opened up and and re-work the ejector and extractor, which it really more of a pain in the ass than what it's worth.
I'd stay with the 30-06 (of course, you can put a short round like the .308 or the .260 in it too) and rebarrel the rifle and have it tuned by someone who knows what they are doing.
Put it in a tactical stock from McMillan or B&C (HS Precision makes a good stock, but I don't care for it's dimensions, it's just an odd feeling stock, IMO).
Hart, Shilen, Krieger, Douglas, Borden, are all good barrels. There isn't any flies on an ER Shaw barrel either, I've heard some very good things about them. I'd go with 22"-23", a heavy barrel contour (if you don't plan on carrying it while hunting, I'd go with a full 1" bull), flute it if you want, and make sure that the crown of the barrel is perfect.
I would go with a fixed power scope, myself. If you're not going to be hunting with it, then fast target acquisition doesn't matter. The Weaver T series scopes are great, Bushnell's fixed 10X Elite can't be beat for the money, and the Super Sniper scopes from SWFA are just fabulous too. These are all generall under $400. If the money isn't an problems, then you're looking at the Burris Tacitcals, Leupold MKIV, or the cream of the crop, Nightforce or the Schmidt & Bender Tacticals (this the new scope for the Marine Corp).
I'd say that 1/2" with factory match ammo is very possible. You aren't going to do better than that unless you handload, really.