As others have said, Mandolin Cafe is the place to go to learn more about mandolins. A new thread pops up almost daily about "which mando should I get for under $500" so the search function should provide you with enough reading to last a while.
I just purchased my first mandolin and am waiting for a Kentucky KM-162 to show up. The main thing to keep in mind is, like with any acoustic instrument, solid wood construction is key. That, and a good setup should afford you a number of good instruments in your price range to learn on until you're ready to splurge on a Collings or F5.