If you're only going to buy one air brush, I'd say to buy the .3mm nozzle/needle and the med. cup.
I have the HP-C Plus, and that size cup, along with the .3mm nozzle/needle works for 99% of my painting. I can put only a little paint in, or more, if I need it, and the cleanup is the same. The medium cup holds plenty of paint for most of my baits.
If I have a lot of single color paint to do, like undercoating multiple swim baits, I use my Badger siphon brush with the .5mm setup. It came with bottles for the paint, and I feel like I could shoot any paint through it.
I have found a smaller nozzle/needle, like the .2mm, requires paint with much finer paint particles, and lots of thinning. Instead, I use an aftermarket inline Iwata air control valve right under the air brush, and am able to cut down on the air and paint flow quickly if I want to do detail work.
Coast Airbrush only shows the option to covert to a .2mm needle on the HP-C Plus, so it looks like that is the air brush that would let you have the option to switch nozzles/needles in the future.
http://www.coastairbrush.com/products.asp?cat=34