1) - Depends where it's from. Amazon, Play-Asia, Play, Any Gaming website you'll be fine with. Ebay is the biggest worry. It would help if you told us where you're getting it from. :)
2) - Yep, Diamond, Pearl, Platinum, Heartgold and Soul Silver are all capable of using the PokeTransfer and yes, two Ds systems are required. They do not have to be the same system. (You can have one Ds Phat and Lite or Ds Lite and Dsi)
3) - Nope, only possible with the Ds systems, emulators are a no-no.
Hope this helps. :)