I'd not get involved in any fancy body/suspension modifications, keep it standard and in good condition.
On a D1, the A-frame and ball joint, radius arm bushes and all that need to be in top condition. Standard road tyres are OK, we use Nexen Roadian A/T's on our D2 and they work well when we have the big trailer on.
Battery and electrics will need some thought as well, more people get stopped by the police for faulty lights than almost any other defects.
We've got a set of Firestone air spring assisters for our D2, fit inside the coil springs at the back and you adjust the lift/assistance by pumping air in or letting it out. They do the kit for both D1 and D2, costs about $100 in the USA, not sure if you can buy them over here. We bought ours online and had our friend in California bring them over on one of his trips.
http://www.summitracing.com/int/parts/fip-4121/overview/
Brakes are fine as standard, but vented front discs and a better than average set of pads might be good insurance, and don't forget the handbrake, that needs to be able to hold the rig in a hill if you have to get out and apply the trailer brake.
Towbar, adjustable is good, but once you've found the sweet spot you'll probably never adjust it again.
3.5 tonnes is a lot of trailer, I know that caterers carry water etc round with them, but a 300Tdi is going to struggle a bit with that kind of load, I know what ours is like with the big trailer on the back and that's a V8.
Trailer tyre pressure and weight balance are critical for happy towing. As with caravans, keep the maximum weight forward of the axle(s) and don't have any large weight behind the axle(s) Towball load is no problem on a Disco, but that's still no excuse to rack it up to the maximum.
Take it easy on the road. We don't have the same effect from a trailer with the big drawbar unit as the weight is squarely on 6 wheels, two at the front and four at the back:
Remember that your Disco brakes need to be in good condition, as until they start to bite and slow the trailer down, the overrun brakes won't do anything, so you'll have nearly 6 tonnes of heavy metal whistling down the road, all relying on those front discs, hence my suggestion to go vented, with some Mintex pads. That's what we use and they have been excellent.
If you're going to tow regularly, I'd think about going for the MTF94 gearbox oil and the semi-synthetic 85W90 axle and gearbox oil. They a bit more expensive, but they are what all D2's have in the manual boxes. If you have an auto box, I'd give it a complete oil change and filter.
Insurance: get the best you can afford. Best not to have anything special in the way of body lifts etc.
That's all I can think of quickly, we enjoy towing our lump, it still gives us a shunt going over bumps as the suspension is trailing arm so there is a slight change of distance between the car and trailer as you go over bumps.
Happy towing!
Peter