I second Plum's remarks about the HP-CS, although as stated before, MikeV's input should be highly regarded. This forum and it's members advice was instrumental to helping me decide on the Iwata.
I agree that the Iwata site is hard to interpret when it comes to comparing different brushes abilities. Like you, I was looking for the smallest line obtainable, and the HP-CS does it for me (and I model in 1/72 and 1/144th!).
The only complaint I might have is that it is a bit difficult to hold, but then I am a carpenter by trade, so my dexterity has diminished somewhat. I'm going to try slipping a larger diameter foam sleeve over the infeed air tube for a larger grip, if I can ever find one.
I also highly recommend DixieArt. Even though my brush was out of stock when I ordered it on a Wednesday, I still received it by express mail (additional cost there) by the following Tuesday. Great people at Dixie!
Just to cover my bases, I also ordered an additional needle of a different size than what it came with (about US$15.00). Would suggest that you get a spare (not necessarily of different size) due to the expense of obtaining one later from the UK. Have also heard good things about Iwata's lubricant.
As far as I know, Iwata (or any good quality brush in that league) can be used on any compressor with a good regulator. My compressor is the same one I use for nailing wood trim in houses! The regulator ranges from 0 to 150 psi. I am looking for a plug-on secondary regulator that will give more control over say a 0 to 50 or 60 psi range. Anyone know where I can find one?
Good luck with your decision, and enjoy your stay in the US!
Stinger