I'm not sure that by changing from a disco to an XJ you're going to be leaving all your problems behind...
I've had my XJ for 5 years now and initially I thought I had made a mistake buying one. Quite a few repairs and electrical issues had me pi$$ed off.
But, touch wood, it's been great for the last 4 years and I thoroughly enjoy going away camping with the family in it.
You may find the ride a bit crude after the disco and a little less cabin room too.
The upside is, they are relatively simple to work on and the mods are limited only by your budget.
I would suggest you keep some of your "purchase" money aside for initial repairs, once you've ironed out the wrinkles then the upgrades/modifications can begin.
Enjoy,
Paul.