Be forewarned once you got to an O/U you might never be satisfied with anything else.
As far as gage choice ,the 12 ga. give you a little more versatility with you shotgun if you want to use it for waterfowl. Today almost all field guns and a number of sporting models come chambered to take a 3" shell although I've never seen the need. Buy what fits ,balances and feels good in your hands. One gets what one pays for. Your 1000 puts you in the used gun market when you are talking a QAULITY shotgun. Many a well loved Brownings and Berettas can had for 1000 or a bit less. You can go with a gun of lesser cost but you get less for your money too and don't let people think you don't. The lower priced shotguns are like comparing a Savage 311 to another double costing $1000 or more.
Choke is up to you. Most guns come with screw in chokes and you can taylor your gun to your shooting abilities and hunting conditions. Fixed chokes go with IC & Mod for a good all round choice.