I would think there is little point in replacing all the friction plates as the box would be faulting into Trans Failsafe if the plates where bad enough to need replacing.
Never use anything but the recommended fluid to the correct specification tin the L322 boxes....
The earlier ZF 5HP26 boxes in the BMW's and the like in the 90's had issues with the clutch cage(?) on the A clutch, the lip/edge wasn't very strong and would break away causing grief..this was changed by the time the box was used in the L322.
The main fault RRPhil has found on these boxes is the bearing between the B and C clutches failing, this cause the clutch pack to move axially along the shaft wearing out the O ring that sits underneath and seals A and B clutches from eachother...the seal failure cause a drop in the hydraulic pressure holding A Clutch in and allows it to slip slightly....
RRPhil reports that the box should fault into Failsafe 5th gear before the A Clutch plates burn out, but it has been known and a search on rangerovers.net will reveal Phil's pictures of shagged A Clutch Plates...
I attach here a couple of RRPhils pictures of the O Ring and Bearing Failure.
RRphil can advise if it is worth while getting it all resealed with new friction plates as he has vastly more knowledge on these boxes than pretty much anyone else...
If it was me, and I had the cash spare - yes I would have it done while the box is out for peace of mind....
Even if I didn't have any issues and I could spare the cash I would get it done in anycase, that is just me being all preventative Maintenance again though!
Cooler, Plate, Stat and Pipes are all readily available (see my How To on replacing the Cooler, Plate and Stat) cost was around £170, And the pipes are both available at about £80 a piece from Island 4x4....
Sometimes I have noticed with Island 4x4 is they may not list it in the menus, but a search on the LR part number usually brings them up....how do I know they are available - cos when I did my cooler I mangaged to break one of them - luckily I fixed it
Parts Required (prices not incl. VAT and from Island4x4 - no affiliation):
1 x Oil Cooler Gearbox / Transmission L322 4.4 V8 Petrol ZF Steptronic £83.33
1 x Thermostat Oil Cooler 5-Speed (BEHR) PBM000010 £30.00
1 x Thermostat Oil Cooler Mounting Plate (OEM) PCU000140 £30.00
Pipes:
http://www.island-4x4.co.uk/cooler-pipe-genuine-ubp000180-p-19737.html - £70 +VAT
http://www.island-4x4.co.uk/cooler-pipe-genuine-ubp000161-p-19736.html - £80 +VAT