I've used a .357 with no problem - after all, archers have been successfully hunting deer (and larger game) for thousands of years with what amounts to a pointy stick, so a modern Magnum will do. Just pick a good bullet, and remember that shot placement is CRITICAL.
For those who recommend a .44 or a .454 . . . sure, why not? Both if these WILL hit harder than a .357, and shot placement isn't QUITE as critical. But you still can't affort to get sloppy with your hits.
Oh, yes, the original question was type of 'scope? It depends where you'll be hunting. I like the Leupold M8 2x. Higher power 'scopes are fine for target work, or if you're in relatively open country, but if you're in rather brushy country, going above 2x tends to reduce the field of view too much.
One of the "red dot" scopes might be OK, too, but being a firm believer in Murphy's Law, I haven't learned to trust batteries in the field yet.