4WD vs 2WD for towing

What is the maximum towing capacity of your purchase? What is the maximum rear axle load? You need to know these before making any decisions. Are those numbers higher in the 4WD version? Based on your figure of 3000kg maximum towing capacity and assuming the quoted ATM of 2250 is correct you would be 750kgs under maximum, which would seem an adequate safety margin and fit for purpose. Perhaps if you repeated here what your advisor said, his comments could be addressed.

Rivers
Depending on what you intend to use the rig for, the MUX 2wd might be quite ok. If ever in wet areas or greasy mud then the 4wd has advantages. The MUX has less towbar overhang than a Dmax so the ball is closer to the rear axle. An advantage there.
If the van has a long A frame it probably won't place too much load on the ball and the rear MUX springs might cope without sagging. To cater for the mass on the towball and also during braking the rear shock absorbers probably won't cope well and a degree of porpoising will be experienced. Isuzu aren't noted for having really good shocks.

I normally towed our 2.2T typical load mass van with our Pajero for a couple of years in 2WD, I never bothered selecting 4H. I don't know anything about MUXs.

AWD or 4WD may be better but by less than a bees antenna unless there is poor traction.
Regrading Law I think Isuzu would have that covered under their claimed towing capacity and the Gross Combination Mass, check out the GCM, for your peace of mind.
Major thing when new to towing is to select the right gear as Isuzu will recommend in their owner manual.

thanks for your input people! I'm planning on adding air bags (?) to the rear suspension to improve drive angle, and as it's a relatively light ball weight I should be good for that too

thanks for that - I'll look into those options

Unless the new generation of the 4wd MUX has gone to an AWD configuration, you would be towing in 2wd anyway, as you would be doing in most other makers twin cab/sw variants with the exception of the Triton/Pajero Sport. Your combination is pretty light in the scheme of vehicle plus caravan rigs, think of all the Holdens and Falcons towing vans around the countryside in the recent past.

The stats look ok for those van specs on the MUX, you wont have any luck returning the car, dont go down that path, they are not in control of your van purchase, you have to do the sums. That said, im sure they will accomodate you upgrading, but you will loose 10k in purchase price.
Power wise you will have ample, you will need load levellers to take weight off rear suspension and rear axle.
If the towball weight is 175, then 350 kg will be on the rear axle without load levellers, and you will be nose up. (this is all rough calcs) leaving 300 kg for occupants and levelling gear and luggage in the car.
Van wise, it seems you will be able to put 500kg in it, water, bedding, clothes food etc, remember 175 goes to the cars weight limit, so even more if packed correctly keeping 175 on the ball.
google your local area for a weigh bridge, reverse the rear on and see what weight is on the rear axle as a starting point.

The 2wd version is OK. It has the same payload and the same GCM as the 4wd version.
The 4wd MUX is "part-time 4wd", so you'd be towing in 2H anyway on bitumen.
If towing on a loose surface - such as gravel, there is some benefit to engaging 4wd.
There has been the odd occasion I've been glad to have a 4wd - particularly if you are taking off on a reasonable slope on a gravel road with the caravan hooked on.
If you bought a different vehicle - say a Prado - it would be full-time 4wd.

thanks again for all your input - good info and in line with what I was thinking