I installed a Wilwood proportioning valve on mine when I installed the rear discs; personally, I don't think there would be much value to it until you do convert the rear to discs.
As far as I know, the dual-diaphragm vacuum booster is as good as it gets. I'm running the City Racer unit, and...
1. It's sufficient, IF it's in really good condition, holds good vacuum.
2. The stock master cylinder will work with rear discs, but they will work a lot better with a master cylinder made for 4-wheel disc brakes. Lots of people (myself included) have successfully used the FZJ80 (non-ABS)...