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There's no reason a Model 70 in .30-'06 shouldn't be capable of shooting fairly well at 500 yards, even with a sporter barrel. 70's have nice, adjustable triggers, and usually have good barrels. '06 can be quite accurate and is still supersonic at 1000 yards with a reasonable range of bullets. A lot of long range matches were won with 70's in .30-'06 way back when. There may be better or more specialized technology out there now, but the old stuff still works.

I'd put money into ammunition components (you'll want to handload because of the control it gives you) and a good scope, then shoot the 70 until I was absolutely convinced that I had really wrung it out. At that point I'd have to look at replacing barrel, stock, scope, etc. versus buying or building something better.