1. I'd go with 2 or 3 of the 2-342's if you have the pump for it, and the 30RZ is the ticket. They are single pass, no need to run parallel, run them in series.
2. 30RZ, no question about it.
3. I see lot's of folks making BADLY designed external boxes. Look how the guys that know build them, Cathar for example. What you want to do is have the box as air tight as possible, you only want holes in the box for the fans/blowers and rads. With the rads, you need to seal them up nice in the box to the opening for the rad(s). Then, you have all the fans/blowers EXHAUSTING out of the box. With this negative pressure, cool air is forced IN over the rads. Obviously the more rads you have in the box, the more fannage you will need for good airflow over the rads. But with 2 or 3 rads, you could have a nice quiet and efficient setup.
*Edit*
Look at Cathar's post, 2nd one:
http://forums.overclockers.com.au/sh...hreadid=300690