You are not going to get into a nice, even decent, SxS quail gun for that money in my opinion.
Not sure what you can get for $600 except for an auto-loader or pump. Many nice small gauge autos and pumps out there I would choose before a cheap O/U or SBS.
Low priced guns are so-so, more money more quality.
One more thing to think about. If you get a good used SxS, and take decent care of it, you will probably never lose a dime should you decide to sell it.
When I read you post, I automatically thought about a SXS and the good ones are pricey.
You are not going to get into a nice, even decent, SxS quail gun for that money in my opinion.
All good advice from people who know what they're talking about. I don't like spending any more money than I have to either, but, if you want a nicely finished, reliable, good handling sxs, you simply have to pony up the coin. I especially agree with colbad (better to get a nice repeater over a mediocre double) and Virginian-in-LA (you'll never lose money on a quality used double purchased right).