Hi @1955W196S!
First post for me on GM. I've been lurking around ever since seeing the journey that mazduce posed about his R63.
I'm an owner of a loaded '12 R350 myself with the 20" factory wheels, P2, etc. I've had it for about 18 months and it also took me about that long to find it. Even then, I had to give up on getting it with a hitch. This because I wanted one with all the interior options too. But know this; you're right. Nothing even comes close to matching the versatility of Merc's station-wagon-on-steroids R Class body style, and it's probably the most under appreciated driver out there.
In my case, I went with a '12 for a couple of reasons. Mainly because it has the updated M276 6 cyl motor instead of the motor that's in all the other 6 cylinder R350's (I don't personally have the courage to go near a used Merc OM642 diesel) . Torque is down about 20% from the R500 but at least it's got the same HP. And I've seen gas mileage between 26-28 mpg if I keep it at 65-70 mph. That, and I think the facelift gives it bit more modern look.
An R63 was just out of my reach, especially considering the not so un-common expense of dealing with a busted head stud, and prematurely worn cams/lifter buckets. As much as I wanted to go there, the premium was just more than I was realistically able to work thru. So I ended up taking some of the difference and picked up a low mileage '06 SLK55 AMG. It's arguably quite a bit quicker stock than a R63. And, I still had more than enough money left over to Kleemanize it. So now it's very fast. Plenty fast enough to keep me watching carefully for the blue flashing lights. But, while now having a sweet '12 R350, and also can enjoy a drop top '55 here in Southeast Florida, there's not an R63 in the garosh. Such is life.
Your lucky that in France the R63 isn't commanding the $30k - $40k premium over the R500 that they have here in the states. And I too am a bit more financially challenged with vehicles than it appears mazduce is (at least it looks like it from the other very sweet performance cars mentioned in the posts, the background cars in the photos of his yard, and in the garosh. Not coveting, just sayin').
All that to say, just as you, I have a trailer to tow too. And now after selling the van I replaced with the R Class, none of my cars (I also have a Prius for commuting) have a hitch. Since Merc doesn't make the hitch any more, I've often wondered how to get a hold of that elusive OE hitch/harness.
Then I discovered that the part number for the harness & hitch was the same for all R Class model years. So while I haven't resigned myself to this yet, and my wife will surely think I'm whacked in the head to move in this direction, I've pretty much surrendered to buying an '06-'07 R350/500 with a hitch on it, remove the hitch & harness, then resell the donor car sans hitch... As you know, there's a lot more of the early model years out there, and if I buy right, it might end up costing me $500 - $750 by the time I'm done dealing with the swap. That's about the only solution I could come up with.
But even the OE hitch on an R is a compromise. Even if you had it mounted to a far more powerful R63, the OE hitch is only a Class 2 with 1 1/4" receiver. Meaning it's not rated for a trailer load over 3500 lbs. There is an aftermarket hitch still made by Curt (PN#13103) that bolts onto the sides of the R's frame rails. And it's a class 3 with a 6000 lb weight carrying load. But you'd have to be able to tolerate that the receiver sits below the bumper. And don't forget you'll still need the OE wiring harness for the trailer lamps to play nice with the Merc electronics.
If you prefer (as I do) the way the OE hitch mounts to the rear of the frame rails & you really needed to pull over 3500 lbs on a regular basis, at your own risk (standard disclaimer) you could have a local fabricator with a good reputation remove the 1 1/4" receiver piece on the OE hitch and weld back a full 2" receiver.
Consider this. A 2019 turbocharged four cylinder Subaru Accent with only 260 hp & 277 ft/lbs torque using a Class 3 receiver hitch is rated to tow 5000 lbs! That's 8 hp less and only 19 ft/lb more torque than a stock, pre '12 R350.
So I'd bet that this same pre '12 R350 loaded with 6 passengers and the trans in sport mode (to get the low first gear) would have more than enough grunt to also pull a 3500 lb trailer. And a stock R500 / '12 R350 should easily be able to handle 5000 lbs with the appropriate hitch. Maybe slightly more. If you were traveling long highway distances with that kind of extra weight on mountainous terrain, it would be pretty easy to add a oil cooler, a trans cooler & a large capacity trans pan. Oil capacity is already something like 8-9 qts.
The other thing to consider is brakes. Obviously the R63 has oversize calipers and rotors. They're actually exactly the same as the similar model year ML63 AMG. So if you were pulling extra weight on a regular basis and found the OE R350 / R500 brakes were fading, those larger ML calipers & rotors will bolt right on. And as it turns out, if you get a R 350/500 with the 7th seat option, you get somewhat better calipers & rotors than what on the car when equipped with the second row console. Go figure. They're nothing like the ML63 calipers, but at least all the rotors are ventilated & a bit more beefier.
Don't hate on me, but dare I say that if towing is a must have ability, once you consider the challenge you have of finding a decent R63 (much less one with a hitch), and deal with the HP tax your stuck with, maybe another option would be a plan B approach. If you don't need to tow heavy loads, find a inexpensive low mileage example of a R350 with a hitch (perhaps 2008 or newer due to the flap valve recall & a few other issues). This gives you the R's super utilitarian body style and the ability to tow. Then use the left over cash to get something else as a driver to play with (you did say you're in the South of France after all).
Or if you need the heavy towing potential and want to combine the extra go, maybe find a pristine example of an R500, again with a OE hitch on it and then have the hitch modified. Explore if it's possible to have a tech you trust fit on some headers & a good air box/filter on to it. Then have a Merc specialty shop recurve the ECU. That would for sure give you some extra go, perhaps 40-50 hp depending on who did the recurve. You'll also find that if you stay rational with the thin petal, those headers would help out the V8 gas mileage too... It ain't an R63, but then you wouldn't have the potential nag in the back of your mind about engine issues sucking excessive funds out of your projected equity appreciation (not to mention your wallet).
Other than that, your playing the R63 waiting game and you might even have to go the donor car route to get a hitch...
If your even thinking twice about this, I'd be happy to show you what to watch for maintenance wise on both the early and late model R's and what to consider replacing as preventative maintenance right after buying one. Good luck and food for thought. Ph4:11