IMO, you should pick up a used Remington 870 Wingmaster with 28" (or 26" - doesn't really matter) barrel, rem choke and 3" chamber. There's no need to go any longer w/modern loads and the appropriate choke. You'll need to grab a late 80's or newer model to get the threaded Rem-Choke that allows you to swap your chokes. It shouldn't cost you more than $500-600, with a few chokes.
If you have zero interest in hunting real birds, you could jump right into an O/U. I wouldn't take a shortcut and buy a low quality one, and unfortunately the decent ones are all expensive, even used ($1500+).