If you are not dead set on an autoloader take a good look at the Remington 870 WIngmaster and Browning BPS. These are two long proven designs and are both reliable and excellent choices for an all around hunting firearm.

As far as the 12 ga.? Would be the way I'd go. A 20 ga. is plenty for hunting small game, wing shooting, turkey and deer and would not be a bad choice. The primary difference between the 12 and 20 is obviously the weight of the load, i.e. amount of shot in the pattern ... and the weight of the slug, should you decide to use it as a slug gun too. You can find a wide variety of shot shell loads fopr a 12 ga. which makes it easy to match load to game being hunted.

If you decide to use your shot gun for deer, buy a second barrel that is rifled. Using modern sabot slugs will give you a deadly combo for deer out to about 150 yards.

Bullcamp makes great point about the length of pull. It is very advantageous to have the length of pull matched to you. Certainly not 100% necessary but in my book one of the most over looked aspects of fit/feel and not that difficult to do.