There is also the Streamline Flattop from Berea. It has metal, quad-start threading, and a plastic nib. PSI's Classic American has a metal nib, but I don't know about the threading.
Seems like most of the nicer kits (Jr. Series, Roman Harvest, Churchill, Majestic (& Jr.)) have plastic threads.
I was given some of the Signature sets in Smitty's swap, and that set it starting to grow on me. That is the same as the Streamline Flat Top from Berea. I don't care for the plastic nib, but then I like the light weight, the look, and the fact that with the quad-start threads it only takes a quarter turn to mount the cap. I also like that they come in Gold Ti, rather than 24K. Smitty sent me fountain pens, but I ordered a Roller ball because I liked it so much. I also ordered a New Series from Exoticblanks.com, but haven't tried that one yet. It only comes in chrome, or upgrade gold, but switched the Streamlines plastic components for metal ones, and added a fancier center band.
ETA:
There are other kits like the Big Ben Roller ball that have snap caps rather than screw. The Big Ben Rollerball is a heavy pen.