Honestly I would say that if you think you are going to stick with bird hunting, go all out early. I have been hunting for 12yrs now, and have been through many shotguns. I finally got a used Benelli SBE1 last year and it has been amazing. I wish that I would have bought one when i first started hunting and then I could have save hundreds of dollars in other shotguns that just didn’t quite do everything that I wanted.
A used SBE 1 synthetic should run around $800.00. I know it sounds like a lot of cash, but if you figure on spending $300.00 to $450.00 this year and then upgrading later on you might as well just bite the bullet now. I have found that if you go to smaller gun shops that have a good client base you often run into good consignment deals. These guns are often nearly new that the owner bought then traded in a few months later for a different firearm. I know of a lot of older guys who rotate guns as often as their toilet paper rolls.
Still this is just an opinion and only worth while if you plan on staying with it. If you buy quality today, you’ll have quality to pass down some day.