I've just put in the Old Man Emu suspension from ARB into my 98 XJ and it has transformed the ride to something completely new, firmer but ride the bumps and dips of the Adelaide hills fine (I'm in Aberfoyle Park) Well worth it in my opinion. I got the kit from Mount Barker 4x4 (Greg there is a jeep fan
). I'd also echo AG's advice on using the cash for an auto-locker to get decent suspension first. You'll find the stock XJ is actually a very capable machine off road and will surprise a lot of people who own the bigger fourbys (Patrols, crusiers etc) on where it can go etc.
I still have the Bridgestone AT's on that I bought it with but I'm looking at either 235's or 30in tyres. Possibly Cooper ST's as I want something a bit more aggressive than a normal AT but don't want to go a full muddy tyre.
If you are new to four wheel driving I'd suggest you go have a look at Saunders Gorge Sanctuary in the hills as it has a nice track you can drive around with a couple of interesting sections to test yourself out on without going to hard at first.
AG - I'll also add Perkins to my contact list just in case I need a mech cheers.