My OH's dad tows a cattle trailer with his, I don't think that he has any problems with it. One thing I have noticed when driving it (not towing) that it wheel spins quite easily and we have difficulty getting it back into two wheel drive
We have a Pajero, which is the same car and a friend has exactly the same car as you.
They are a really good car. Like any diesel have the fuel filter looked at / changed, as this can cause troublesome starting / running sometimes.
We have found ours to be totally reliable and have towed two horses from time to time, our friend has her own trailer and whizzes about all over the place with a variety of horses and ponies. She recently brought our latest addition (14.2 New Forest) back from Swindon to Somerset easily.
For parts Mitsubishi are expensive. We get our bits from Milner Off Road, who have a web site. That said, we have only changed a couple of filters and some windscreen wiper blades in about 4 years of ownership and ours is used every day.
I have J reg Mitsi Shogun 2.5 TD, done 186k miles, and although it has cost me in welding, it tows so well. Had a Disco 3.5L before that and Shogun is far superior.