Simon, there are a lot of good airbrushes out there for camo... as you are about to find out when the airbrush wolves get here...
If you do decide to get a 155 you've got all you need right there... there is only one size needle for the 155... it's not a medium. more like in between the medium and the fine the other badgers have. I'm able to do a pretty good squiggle camo pattern when practicing (too bad I am trying for straight lines) down in the 1/16th of an inch range more or less (haven't measured it but they're some pretty fine lines!... the needle with the 155 allows you to use model paints without over thinning like I have heard the fine for some of the other brushes require...
Other good brushes like the 155 are the Omni 4000 and Iwata HP-CR. it's all how much you want to pay and what you decide is the best for you. The Iwata HP-CR has the same one size fits all needle/nozzle combo, not sure about the Omni 4000 though I think it does too.
The interesting thing about the Iwata and Omni is that they are gravity fed which means less pressure is needed to get the paint to the tip, which means easier fine lines without throwing too much paint down... the 155 with the metal paint cup mounted has done me more than fine, so I am happy with what I have... although I do want to try a gravity feed the next airbrush I get... the thing I like about the 155 is it takes only about a minute to clean...