I just scored a dozen tubes of the MB summer stuff from "captruff", it's the only thing I use. At that price I pay far less than the dealer wholesale cost and I have a bunch on the shelf for whenever I need them.
You will have to disconnect the cooling system hoses to the "heater" pipe to replace the rubber grommets, but it's an easy job. As long as you release the pressure from the cooling system beforehand you won't lose any coolant to speak of. Might also want to get a new reservoir cap, too. They're about $4.00 and are simple to replace. I'm betting the retention strap on yours is probably gone if it's the original - they all tend to break off over the years. Grommet part number is 010 997 13 81 I believe.
While you're doing this I would also suggest replacing the grommets on the windshield washer and headlight washer pumps. If they're not leaking now they will be in the future. Be very, very careful removing the pumps - it's easy to do and there's a technique - you push the pump body straight up while it's in place in the recess on the reservoir. DO NOT "TIP" IT OUT!!" I say this because it's what most people try to do when removing the pump. This approach will snap the "nose" of the pump off in the grommet and render your pump useless.
If you push the pump straight up in the recess where it's held in place, the nose of the pump will come out of the grommet and clear it. At that point you can tip it out of the recess without damage. The pump is held in place by the physical pressure of the reservoir, so you'll just pull or pry on the top of it and it will come out. Installation is the reverse of removal - snap the pump into the recess, minding the position of the pump outlet that projects out of the pump body. At this point I like to make sure the "nose" of the pump has some water or other lubricant on it so it will slide easily into the grommet. Push the pump straight down to engage the nose with the grommet in the reservoir housing and you're done.
The grommet is a Mercedes part number 123 997 36 81 and will set you back around $2.00 list, far less if you buy it from an online dealer. DO NOT buy aftermarket versions of this part! You'll need two of them, one for each pump. I usually keep some on the shelf, as they fit literally every model Mercedes ever made as far as I know.
This is an easy and very satisfying job to do, as seeing the ratty grommets around those heater lines always bugs me.
Dan