Mmk, I might as well weigh in to put all of the answers you may have in to one concise area (other than of course the stickies, which do that as well).
Most 3ds systems that are out now will probably have a firmware higher than 4.5. That being said, my sister got a pink XL for her birthday running on 4.5. You need to check out which firmware you end up getting first before you proceed to determining whether a Gateway is going to be an appropriate
IF you have a firmware version of 4.5 or lower, then you may wish to purchase a Gateway. The Gateway will play games requiring up to 4.5. It will also "spoof" firmware, making games that are higher than 4.5 essentially think you are on a higher firmware. Games currently out will likely be fooled by this spoof. Animal Crossing: New Leaf will not work, however, because that game is different than most, and has additional hardware (an onboard NAND save on the card itself). It's kind of like when Pokemon Heart Gold and Soul Silver came out...that card had an actual infra-red receiver on the card, so if you pirated the game you had no way to use the Pokewalker even if you purchased it separately, since you had to make it talk back and forth with the game cartridge and a CycloDS, Acekard etc. doesn't have the IR port to do so.
Now to answer your Pokemon question: we don't know. It's possible that the game is so far along on its current tested firmware (which is probably 5.x or higher) that they won't have time to make it require a brand new firmware (because yes, we can fool the Gateway into thinking you are on a higher firmware, but once Nintendo figures out how they will, or maybe already have, try to design games to detect and stop the spoof).
So my advice: wait till you get your 3ds to decide. Once you've got it, you may want to wait till Pokemon comes out to see if it is able to be run on the gateway with the current spoof available. It's also possible that in that time a new exploit will come out for the Gateway making it playable. Once you have that info you can make a more informed decision.
Also if I've gotten anything wrong feel free to correct me