Under $300 used is decidedly limiting. Highly unlikely you'll find a semi with all the extra bits you want for that. Wrong time of year to be shopping for used firearms too. After hunting season is the best time for used stuff.
There, currently, is 1 Rem 1100 on Gunbroker with changeable chokes(very good thing) running $335.90. Rest of 'em are starting at about $800. 11-87's start at about $600.
A rifle sighted barrel runs about that BNIB. There's a rifled barrel on Guns America at $250 though.
However, there are a few off brand/odd brand but serviceable semi's on the assorted auction sites. Issue will be finding barrels and choke tubes. That's something you don't need to do immediately though.
You don't really, absolutely, need a 3" for turkey or geese. 2.75" magnum with a tight choke will do.
If there's a Gander Mountain near you go have a look. Their site shows something called an 'Escort' semi for $299.99. Never heard of the brand myself. Apparently decent kit made in Turkey and imported by Legacy Sports.