Well, where to start?
I presume it's a standard road Disco?
First things first, Tyres - might not need full on Mud's, but suggest at least a set of all Terrains would be sensible.
As far as the rest, depends how much your wife will let you spend! get the things from Tom's list first, then think about tyres, then you can start to think about the extra nice bits. That said, i think most of us have CB, so that'll prob be really useful, and you can get a full kit for about £100.
Hope that helps a bit, just be sensible, as you can spend thosands.
Go for the basics first, i'm Jon from Venture Overland can be of serivce to you....