I have used all types of shotgun action types and currently use a Beretta O/U Water Fowler, that has a heavey coating on the metal from the factory (pre-CeraKote days) and the stock has been bent/fitted per my shooting try-gun specs. My backup is a Benelli M1.
Whatever you decide, I would recommend it have a rib on the
barrel, as it provides a much improved/easier sighting plane. Also, if you go with an 870, select a model with some action smoothing, not the "Express". I do not know the name or model #. Research the model and look for a used one. Also, any used 870 will be smoother than a new one, not just from quality issues, but from repeated cycling. It will add reliability and enjoyment. Not an abused beater, but maybe a few years old. Your practicing with a new one will smooth out a new one as well. Synthetic stock has been mentioned and I agree with that, again for reliability and enjoyment.
The one advantage to a 3 1/2" chamber is that steel shells are less expensive than the advanced technology non-steel 3" shells. Compare ammo costs before deciding on chamber. 3" 870's will be less money than 3 1/2" ones. Just bringing this up to accomodate budget decisions.
A floating fabric gun case is nice. End of season sale. Jan/Feb is a good time to purchase firearms.
Practicing with clay targets thrown by machines is fun and repeatable. It costs and may be far from home. A group of friends and a plastic hand thrower is easy and also fun. Quick and easy to change angles. Also, some missed clay targets can be reused. Throwing is fun, rotate through the group. If the field will be cultivated, no problem. Otherwise, look for clay targets that will break down quickly/quicker.