Most all of them have 80,000-121,000+ miles. Very few with actual advertised mileage after verification.
Good usable low mileage V8 AMG nearly costs more than full rebuilt 3.2. Then you gotta keep buying parts to complete install. Adding 200# to the front of a nose heavy car is counterproductive. Unless 1/4 mile is your poison.
At that rate, just buy an SL55, bolt on go fast parts and enjoy! To each their own. V8 SRT would be fun, OTOH.
In my opinion, the m112 and m113 are the last of the bulletproof easy to work on engines from MB. Compared to the new engines that MB makes now, the m112/m113 are as simple as a push rod SBC.
I would not want a sl55 to much electronic crap to fail. One of the key things I like about the srt-6 is the fact that it is a simple car without all the unnecessary MB crap. Meaning no ABC for suspension, no air ride stuff that WILL fail and cost an arm and a leg to replace.