RE: Groundhog Rifle
If you are starting at 500 and going past, you need to start in the 6mm class and go up. A 243 is a good choice with 100gr+ heads. The 6.5x47 is getting a lot of great reviews by the long range target guys. The 260 rem would be a good choice and the 25-06 wouldn't do bad either.
You're going to have to reload obviously, not just for consistency but for the higher BC bullets you'll want to be shooting. Depending on the rifle it may end up being a single shot as the heavy bullets get long for magazines.
Which particlar gun? If you want a stock rifle then the savage is hard to beat. The problem will be twist rates. Might not get a fast enough twist for the heavier heads. Barrel changes are easy though. If you want to customize then the options are limitless. I'd probably start with a savage action and build up.
Scopes are personal preference, but buy quality. I like the bushnell elites in their price bracket. Very clear, good image, good brightness.