they can be heavily overpriced from some of the more common retailers... You will struggle to beat these prices-
http://www.machineryhouse.com.au/Castor-Wheel
Thats Bozo's(Sam) Z. IIRC he paid 6K for it not to long ago. He also hit something damaging atleast the bonnet, not sure how it was repaired. Would be a good base... but 16K is a little much.
Nice work Gav, though I cant help but think you should remove the rest of the skin properly too as to ensure its 100% rust free. Seems like and awful lot of work for the rest of the frame to "pop out" one day.
Interesting, would have thought that minimising the amount of flexible line would be a priority. Whilst convenient wouldn't it have a change in pedal feel due to the slight expansion of hose under pressure?
Z-Shop seems to be $295 on ebay?
http://www.ebay.com.au/itm/240Z-510-1600-180B-SSS-240K-7-Brake-booster-rubber-diaphragm-Datsun-Nissan-/281595508922
edit: that may not include the rubber diaphragm?
IIRC the flange box is the best one to get in the "standard" range as it has a fairly CR gearset. Someone will have a tailshaft kicking about just put up a WTB.