.177/4.5mm is fine but if you want to do that you should know you can't play with it with us only target shoot and whatever.
Also make sure it's a 495 FPS version otherwise you'll need a PAL for the 500+ FPS versions.
Also Airgun forum is probably more the place for that kind of stuff, they know more about that than we do since this is an airsoft forum (granted I do know some stuff about .177 airguns but nothing in comparison to the experts there):
Also why not just go here if you're looking for a .177 airgun, much easier and a LOT less hassle: http://www.dlairgun.com/
EDIT: Highly recommend the Beretta CX4 Storm for some fun, or the Daisy 853C Legend if you want to get your feet wet in some serious competition shooting.