Hi Paul
I tow a Bailey Ranger 510/4 with my 2002 Zafira 1.8 Elegance (125bhp). It will cruise at 55-60 on the Motorway with no problem , you certainly notice the hills though ! and you can find yourself towing in 4th a lot if it is breezy as it seem sto struggle with 5th , only had this a couple of times.
The problem with the Diesel is that it has a low max tom limit of 1050Kg , whereas the 1.8 Petrol has a max tow limit of 1300Kg , although even this limits choice when looking for a 4-berth family van with an end bathroom !