There was never a MK4 road car with P/S to my knowledge, i have heard the mobility ones came with P/S but i can't see Ford developing power steering to fit onto a model they only sold a handful of!

Mk5 cab's had power steering though i believe........

More than likely retro fitted by mobility themselves as you say, but either way it could still be sued if you can find one (good luck )

And above all......is it really worth it, the steering isnt THAT heavy, and when you lower and put bigger wheels on the steering becomes very light anyway