Well, since this thing is still going on, I guess I'll jump in. Limiting oneself to 16 or 20 gauge for $500 probably means either an American hardware store gun or an obscure Spanish double. A few years ago I bought an Aramberri 20 with PG/DT/beavertail and 28" vent rib barrels, in very decent condition, for $375 on GunBroker. Just before that I grabbed a nice Armas Erbi 28 ga with PG/DT/splinter for $700 on AuctionArms. Was not familiar with either of those makers before the purchases, but learned as much as I could before making my bids. Did not get burned on either one. If you go that route, it's worth investing in a used copy of Weiland's "Spanish Best," just for the references to the obscure and defunct Basque makers, such as Gorosabel.
Having said that, I agree that a first double should probably be a light 12, because of price, availability, and flexibility.
