The towing capacity for a 4WD is 3300lb braked, 1300lb unbraked. 130lb maximum hitch weight. (Source:
NZ Automobile Association list)
(Note: Weight in kg. The L300 had a facelift in 1994, but the pre-facelift 4WD continued to be sold until 2000)
However- towing capacity calculation is an odd thing, and any one factor can cause the manufacturer to limit it: it might be chassis strength, vehicle weight, a weak gearbox, inadequate engine power, or extra wear and tear leading to an unacceptable number of warranty claims.
I'd say the L300's weak points are chassis strength (the chassis behind the rear axle isn't particularly strong, especially the crossmember/valence behind the bumper where most trailer hitches are attached, which explains the low hitch weight). If you're installing a trailer hitch make sure it ties into the chassis rails just ahead of the leaf spring shackle attachment points, not just the rear crossmember.
I tried to purchase an off-the-shelf hitch, but all the ones I found were a bit weak looking, or bolted to the towing/tie-down points, or only had a couple of attachment points. I ended up just building my own
(Google Photos album here)
The L300's short wheelbase can be an issue if you're towing something long with the weight at the ends rather than over the trailer's axle- a long trailer can shove an L300 around if it's improperly loaded.
The L300 4WD certainly weighs enough to tow 3500lb safely, and the drivetrain is strong enough as long as you don't use overdrive (auto) or 5th gear (manual) while towing.
I have fitted air adjustable shock absorbers to mine, and they're brilliant for levelling the back end when the trailer is attached.
My caravan only weighs 2200lb, plus another 1100lb of gear, food, water, and beer in the Delica. The original, tired engine was a wee bit slow. It would do 60mph, but it was a bit hotter than I liked, and it wouldn't maintain 60 up hills. 55mph was better. The new Hyundai D4BF engine is fine- still no speed demon when towing, but it'll hold 60mph all day, albeit at 3500rpm in 4th. The caravan doesn't push the delica around at all.
I've towed trailers full of sand/gravel/dirt (about 4400lb) and while nothing immediately exploded, the weight was far more noticeable, shoving the van around quite a lot. 50mph was comfortable. I managed 60mph (eventually, and briefly) but the handling wasn't confidence inspiring.