Hyper, my recommendation would be to get HeartGold or SoulSilver. They're remakes of the second generation of pokemon (silver and gold), and are my favorite in the entirety of the handheld series by far. Black and White are outstanding, but the ones I've mentioned may be the most familiar place to jump in.
Also, it doesn't matter if you get it pre-owned, but you'll have to look up instructions on how to delete the file already there (there are not multiple save files, much like the first games). The instructions are a little weird--you basically have to push some button combination at the start screen or something like that. Google will know.
If you get the games and have Wifi we should do some battles!