Hi! Chris,
I too have a 2.5 Na and was happy a fellow forum member was breaking down his Defender and I bought the whole PAS stuff from him.
Its a joy now to drive the defender, without PAS it was very very painful in Indian traffic conditions. While driving its ok but when you need to park, its a nice gym workout for the arms and shoulders.
LANDY POWER ASSISTED STEERING:-
Yes you can convert to PAS, you need to get hold of:
A Power Steering Pump
A Twin belt crank pulley
A PAS steering box
The pipework required, and
A fluid reservoir. It isn't too hard, but you gotta get these.
Oh, and another belt.
You should also check that your engine has the holes dirilled and tapped in the side of the block for the pump - they mount in the bottom right hand side, as you are standing in front looking at the engine, underneath the alternator.
Try asking on the forums. You might get lucky, just like me.
some more pics on the pipes you will need
fluid tank
PS box