I've been curious at just how capable the 250d would be with towing and after reading Motoring's tow test article, it doesn't seem like its much of an improvement over the capability found in the Navara. Motoring gave it a 72 out of 100 but criticized it for not really providing better performance, fuel consumption or stability than what the Navara delivers. With its current powertrain they said its not an ideal heavy duty tow vehicle, and that it may be better to wait for the V6. Check out the article here.
It looks like most of these shortcomings have been improved upon in the V6 model and MB has even outlined the possibility of a V8. Though thats not going to help with fuel consumption. MB should consider some sort of mild hybrid system.
Tuners can help here and I will be looking out for that although my towing needs are so minimal a GLE can do it. Still helps to have an overkill of lower end torque especially moving from a stand still on an incline.
Yeah I think its worth considering on what exactly your needs are with the truck. As the majority of those with an interest in the X Class probably wont be using it as a proper work truck, the 250d may be completely satisfactory for their use.