Driving a 2010 ML350 Bluetec ... diesel is the best thing ever for an SUV ... getting easily 20 - 22mpg in town and 30+ on the freeway ... towing capacity is 7000lbs. with the original MB hitch - not sure how yours is equipped. When towing boat it goes down to about 15mpg - not too bad either but I don't have to tow too often anymore. And you have no problem with keeping up with the traffic - has plenty of torque ;-) ...
If you look up forums - some complain/have issues with the tongue load when towing large campers - not an issue with boats as the weight is in the back anyway. Maintenance/service is not too bad ... yeah you may pay a bit extra and just in case - bluetec as run-flat tires due to the used up space for the adblue - so no spare - those do run more expensive when you have to replace them ... will know in a few months when mine are due. Free loaners when you bring it in and no need to book 10 weeks in advance to get one ... so you get a few things for the money
one thing to know upfront: Darn thing hates LED trailer lights - don't have enough resistance so warning/malfunction light goes off if you don't use an aftermarket resistor between truck and trailer ... VW has the same issue.
Not sure if you by certified pre-owned or not - but if you do - warranty is usually 100K ...
Can't speak for the GL but remember when buying this one ... stood in the showroom next to a GL450 - gas mileage (talking gas not diesel) said arond 11mpg (I'm like 'ok - if you can afford a 90K car I guess you don't really care about gas') - so diesel will save you a few but it's a big car and if you don't have a bunch of kids, dogs, one wife and what else you need to fit ... doubt you really get the savings - especially when you drive around by yourself 90% of the time ...