Not too tough
The job to replace the tensioner is not terrifically difficult, though it requires small hands and some finesse. I have the 2.6 liter on a 124 car, so my pointers may or may not apply to you.
Some of the more difficult points are getting the center bolt out of the fan clutch, and getting the lower bolt and nut off of the little dampener shock.
My dampener shock (incorrect part designation) had a nut and bolt combination. The new tensioner (if you get the aftermarket one from Mercedesshop.com) does not use the nut, so reinstallation was easier. If you find that you have a nut on the back of that, wait to remove it after you remove the tensioner.
You will need some specific instruction for aligning the tension pointer and sealing one bolt on the fromt of the engine with sealant. I would have never known to do that, had I not purchased the Popular Mechanics repair CD. (I recommend that, by the way, though it does not cover some topics.)
The fan clutch bolt is almost impossible to remove unless you know the "trick" to immobilize the fan shaft. I can not adequately explain it here, though note that I was able to use a small allen wrench, in lieu of the specialized MB tool.
Parts, as I recall were about $150, but you might ask the pros to offer insight on what else you should replace while you have the front of your engine taken apart.
Time was about 4 hours for me, though a pro could easily do it in two.