I think 'Shanks' was selling a centre one-way pulley a few weeks ago.
Maybe PM the chap?
Ohm...
I started with a standard 501x, and slowly added the WE parts; it's mainly the big pulleys, alloy uprights and then the shock shafts. I can't immediately think of anything else. (I believe the HL dampers are an extra optional even for the WE.) It hasn't really cost much, as most of the parts can be obtained from Japan dirt-cheap.
There's a brilliant 501x review by Jimmy that's worth a look:
http://www.oople.com/rc/photos/review501x/
Also a write-up about the one-way and pulley upgrade:
http://www.oople.com/rc/photos/cars/501x/bigbelts/
There's a lot of people that still run the 501x as the only major difference between the 501x and 511x is the ease of maintenance due to the revised bulkhead and top-deck configuration. Best thing is to read through some of the threads in the Tamiya section of this forum, and if you have any questions then just ask...
Julius.