The quality of the glass means everything to start with. If you buy a low end scope you can get so much diffusion that you cant see the target clearly at any range. On the other hand you will rarely see anything clearly at 1000 yards anyway due to mirage, but that does not build a case for low end glass.
Sheephunter is right that the target you are trying to see is a factor. For years, the US sniper rifles used fixed 10 power scopes and they seem to have done well enough. Lately though they are using more.
The trend in F Class is more magnification seems to be better, although I seem to do well enough with a 6.5-20 power scope. Lots of the guys seem to like 30-50 power scopes.
I would think you should be looking at something like 6-24 or 8-32 like the SIII series from Sightron, but in the end you just gotta go with what ya can afford.
One more point is that the travel limits go down as the magnification goes up, so that might be reason to avoid some higher magnification models. Just keep an eye on the travel limits to be sure you can get where you need to.