Actually, you are in luck as I just replaced the COMAND unit in my dads 2001 S500!
FYI: you do not need to get new speakers and the rumors of the bizarre impedance are from somewhere on something. I did not encounter it. I think people used a multimeter to measure the current speakers. This will not give you the actual speaker impedance but the speaker DC resistance. There is a a difference and its an important one. As, a speaker may be rated for an impedance of say, 8 ohms, but a multimeter will show 6.3 Ohm. Why? The impedance is measured with a frequency injected in order to have both capacitive and inductive reactances generated, which then, after an equation, will yield the "impedance". Typically, a speaker/driver is measured at 1kHz. But, some manufacturers provide a "sweep" chart.
FYI2: A car driver will typically have an impedance of 4 ohms. So, a "bizarre" DC resistance of around 2, maybe lower if its a tweeter, would not be unusual.
This is a serious undertaking and most certainly not for the faint of heart. If this is your first car-radio install, I suggest budgeting about four weekends to complete the job. Seriously. It's that involved.
We replaced the COMAND with a Pioneer X930BT, Pioneer GEX-P920XM, and a Rockford Fosgate R600-5, CANPULSE2. If you need info, look the bits up on Amazon. But, I will have some rationalizations after the "steps".
Any ways, I will give a brief breakdown of what we did below, "days" are 8 hours (+/-):
1) The first thing we did was safely remove the COMAND system up front, and then the associated bits in the rear: disc-changer, amp, etc. This is a lot of work as you have to remove the interior of the trunk and take it all out. About a day.
2) Ran new audio cables to the rear. This was a big job as removing the interior panels can be tricky as you don't want to break the retention tabs. Also, getting the cables into the trunk was also tricky and we had to use a line to pull them through. Half-day.
3) Fabricated a bracket to mount he Rockford Fosgate amp. It's much bigger than the original one. We chose to mount it on the corssbrace that is between the gas tank and trunk. This took awhile as the determination of where to put it, plus removing the crossbrace, modifying it with the amp mount, test fits, and reintsall. Took about a day.
4) Had to find all the signals that I needed for the amp. This was a weekend in itself as I had to find the info online. Then, read all the schematics I could find to determine what was what to do what I needed it to do. Two-days.
5) Make all the wire looms for the new system. Another weekend. This is not easy as I had to unwrap the existing wire looms, separate specific wires that I needed, and then recombine them. Plus, I wanted to make it look "stock" so I bought cloth friction tape to wrap the new looms with. Looks awesome. Two-days.
6) Starting to install it all: first the amp. It will not fit like you thought and you will have to back and forth to make it fit. Plus, routing the wires to make it look decent. A day.
7) Head unit install. Very complicated. Since you will have to route the GPS antenna, XM antenna if you want XM, external mic (can't use the stock one), make space for additional cable bundles required for iPhone interface, XM radio, etc. I'd say two-days, maybe more, I forgot, but its a PITA to reroute all new wires for antennas and what not.
8) NOT YET COMPLETED: Install the SWI-PS to utilize the steering wheel control buttons. Probably a day as I will have to take out the X930BT to interface to it.
How does it currently sound: Awesome. Much better than than the stock COMAND and amp. Literally blew me away while I was driving it. Brought a smile and giggle and made the whole insane effort worth it.
Pioneer X930BT: Has bluetooth, decent nav system, recent so things like XM and traffic radio add-on's are actually available. Also supports external inputs and an external monitor. Also, you can choose your button colors! I was able to make it match the red-orange of the S500 almost exactly! Oh, also has a removable screen. $520 plus $100 for the XM.
Rockford Fostgate amp: It can drive a 2 ohm speaker, has five channels (four channels plus sub), and is about $200.
I am glad to hear about the speakers only being a rumor, and I want to elaborate I don't intend any disrespect, but I have to greatly disagree on your times.. Maybe you have never done a car stereo before but removing the head unit (Comand) infront takes about 4-5 minutes, it's four screws. Remvoing the amp/CDC/PSE/VR in the trunk is maybe 10 minutes max, not a day! You only need to remove the trim around the CDC, which just pops out, then two plastic clips and one screw for an interior hook and the whole side of the trunk lining comes right out, then there are five screws holding everything in. Runing the lines from front to back again is very simple. You run them from the drivers side door sils. Pop off the plastic pieces and run them under the carpet where it meets the side of the car, they can easily be fished through to the back with a coat hanger from the side of the seat. Push up on the seat firmly while pushing the wires under it, then pop back on the plastic door jam covers to hide them.. make sure the wires don't get caught in the clips! Running the amp wires will be super easy becasue the battery is in the trunk.. ten minutes tops.. the RCA's and amp wakeup can do down the side like I stated.. the wires for the stereo can be taken from the factory Comand power plug, the canbus wires are in the larger connector (NOT the largest connector). Setting up the can adapter for wheel controls can go very quickly or sometimes be a bear.. tops an hour with trouble.. I am sorry, again I don't mean any disrespect but this job is a one day job for someone who has atleast put in a few systems.. even if you have never done one before I couldn't see how it could take longer than a weekend..
I have seen a few systems go into these cars and helped with one, not including my own.. it's actually easier than most cars because the battery is in the trunk, you don't have to run that big fat amp cable through the car..
Also, XM antenna can be ran under the dash, on top of dash, up a pillar and two sided taped behind rear view mirror, or ran down side of car like above and velcroed to accesory tray behind rear seat.. that is where I have mine.