Auto Navs aren't so thirsty that they should be discounted, and if you're doing some heavy towing, the auto seems to be preferred by dealers & caravan sellers, including Geoff Dutch, a Newcastle truck body manufacturer who now also builds the Travelhome fifth wheelers.
With that much weight on your towball I'd look at a load leveller (I use one, with my 1800Kg caravan = 180Kg ball weight, or 10% of towed mass). The load leveller helps push the weight back onto the front wheels. Stiffer suspension won't do this - in fact, stiffer suspension will only help make the front wheels lighter, and in the wet that can spell disaster. (please note, this is to help everyone reading - you may have experience towing, but you didn't say you were using a load leveller).
Higher-rated rear suspension is essential anyway, and you may be persuaded by the suspension people to look at air bags - don't discount them immediately, many people say lots of good things about them.
As for whether Thai ones are better than Spanish - who knows for sure until we sit several side by side and go over them with fine tooth combs. Nissan will provide specifications to the factory and the factory has to adhere to those specs. It doesn't matter if the factory is in Thailand, Spain or sitting on a rock on the top of the Andes ranges in Brazil. The vehicles ought to be very, very similar.
There's a thread hereabouts somewhere regarding the DP chip. Don't jump in and buy one until you've had a good read of it ...
click here to read the thread.
I just remembered something important: if you are using load levellers, don't use the genuine Nissan hitch, get them to mount a Hayman Reese hitch. They bolt on differently - the Nissan might take 3,000kg, but won't take the twisting effect the load leveller places on the hitch.
