I was going to suggest a USFA revolver such as a Rodeo. They were under $600 each when I bought a pair of them from longhunter less than 3 years ago. Now they're up to $750 each!
A used HK P7.
Where can I get one in good condition for 600? I would DEFINITELY consider that.
I'm leaning toward picking up a P99 right now, actually, because it's a "real" gun to me. My collection is slanted too heavily toward milsurp at the moment.
If I was just going to buy a gun for the heck of it I would get the Colt 1903 in 32 cal. Every gun lover should have one. It is a very fun gun to shoot. And their is a lot of history in this little gun. JMB was way ahead of his time with this one. And it is all steel. If not that than it would be the Kahr K elite series.