The only type of hitch you're going to find is a bolt on receiver hitch. I researched this quite thoroughly last summer.
Call Da'Lan Hitch, Inc.
Walker Road, Shirley, MA 01464
(no web site)
They make a receiver type hitch that will fit the 300E. I installed one last summer to tow a 10' pop-up trailer and it worked like a champ. It cost me $273.65 ($237.05 + $36.45 freight) delivered to the L.A. area. I installed it myself in a couple of hours. It requires getting the car up on jackstands, crawling underneath and drilling some holes. Just be sure to measure twice and drill once! It uses reinforcing plates inside the trunk to add strength. I had to loosen the muffler hanger mounts and get my son to pull it off to one side in order to get one of the bolts in, but otherwise it was pretty straightforward. Because it's bolted up against sheetmetal that's undercoated it might loosen slightly as the undercoat compresses, but I only had to retighten 2 bolts one time on a 2700 mile trip. The hitch is well-made, strong and relatively light. Once it's in, it totally unobtrusive, although you will lose a little ground clearance heading out a driveway with a steep drop to the street (but backing out should be no problem) Just make sure the trailer light wiring connector is up out of the way. When it's installed all you see from the back is the square receiver tube just visible under the rear bumper and inside the trunk you see nothing other than the wiring for the trailer lights. In order to run trailer lights though, you need to get a tail light power converter. Hoppy makes a good one, available from any good trailer/RV supply store. Check this link:
You need to run a 10 or 12 ga. wire from the battery + terminal back to the trunk and wire it up. You can run the wire from the battery through the firewall into the passenger side, down along the front and rear doorsills, up and through the rear bulkhead behind the rear seat and into the trunk. I'm not sure what a shop would charge but perhaps Da'lan could give you some idea of labor time. Figure probably an hour for the wiring.
Hope this helps