I can't remember who adopted the slogan "Just Do It" for their product, Nike maybe?
Anyway, that's my opinion on the questions you asked. Both are not only practical and doable, but have been done.
The Disc conversion is fairly straightforward, and a few companies offer a package, Rovers North for one, here in the US.
The PAS is a little more time consuming, but no more difficult than the disc job. You need to get hold of the componants from a Defender, and most should be available second hand, though I'd start with a fresh rebuilt box and new pump, only because they're a PITA to get out later.
You will need to ditch the series column and get a defender column and breakaway section to replace it with.
There won't be the appropriate holes through the frame to bolt up the box, so you will need to get them drilled and weld in some sleeves there (so you don't simply crush the frame box when tightening up the bolts)
Since the drop arm from a PAS box is different than the lower arm off your relay, you'll also need to change out the drag link, no problem there either.
Do it, photo log it along the way, and post it up here when you're done.
