As the OP on this thread, I wanted to thank everyone for their input. The 510, 564, and 590 are the top contenders for my first Panerai, assuming I buy new. A sandwich dial 177 is a bit of a grail for me, but I can see those don't come up too often. Even the newer 8-day movement watches are tough to find with sandwich dials at this point.
In truth, all of these are nice watches to my eye -- painted or sandwich. Since half the fun of this hobby is the anticipation I may wait a bit longer to see of anything turns up. But a new, painted dial 564, 590, or 510 would be a keeper, I'm sure.
John