Hey Sooty, Just in case it helps,
I bought a T28 off a S14 Sr20Det, it's the bush bearing one not the ball bearing one (*cringe* more lag for me), but it was in really good condition, practically no shaft play, all seals and bearings in good nick, seemed too good to be true. Paid 400 all up. There are lots on the silvia forum and even a couple on Ozdat.

Oh, and the engine guy said that keeping the engine stock (with stock boost) and swapping the T25 to a T28 should happily make at least 150 kw at the wheels (not sure if he said a high flowed T28 or not). Again, people say lots of things before you give them your money so not sure how accurate that is. I'll let you know as soon as I find out though.