I would start out with a "Low Mileage" (most, are) 12ga. Auto, or even a pump, and start looking for interchangeable barrels. After a few months you will find that your interests will take you in one direction, and you will be able to decide for yourself.
Right now I have a Wingmaster with barrels from 18" to 34", some with interchangeable chokes, some with fixed chokes. Also there are 3 or 4 different "Corncobs" and as many different butt stocks. Overtime, it has become my "Loaner" to friends and family.
The 870 with a 30" barrel and a homemade (With cardboard & duct tape) trap stock was my first trap gun. Now I use a Browning Trap Spe., for 16 yards, Doubles, and Hdicap.
The 870 with a 26" was my first skeet gun. Now I use a Browning O/U with 28" barrels.
The 870 with an 18" barrel was my first "3 Gun" shotgun. Now I use a Benelli M1 Super 90.
The only hunting I do is coyote, and again the 870 was my first coyote gun. Now the Benelli with a 26" barrel is the coyote getter.
Side Issues:
If you reload, the auto will throw your shells into a pile 7' to 8' away on the ground, and you will need to pick them up. The O/U, you just pull them out of the chamber and drop them into your pocket, and move on. The O/U will shoot any load that will pattern, and the auto will shoot almost any load too.
With the auto, you put one choke in the barrel, and use it for all the shots at that station. The O/U, you will put one choke in the "Under" barrel for your first shot, and a second choke in the "Upper" for your second shot.
If anyone complains about hitting them, with your sent shells, just tell them to "Move their Fat A$$ back to where they should be standing, when its your turn to shoot!"