I faced the exact same dilemma a few years ago with a heavy duck model 12. I wasn't going to alter the choke, and couldn't afford a steady diet of botique shells. It was never going to replace my 870 anyway. I ended up selling it for a modest profit. I do miss it sometimes though. I think my model 12 itch would better be satisfied with an open choke 20 (or maybe a 16) for the uplands.
As far as what you can shoot, bismuth or heavy shot are about it. Looks like the price has come down though! Midway shows $1.60-$1.80 a round (though currently out of stock).
You could always repurpose it as a turkey gun and shoot lead.
