A bunch more information:
My transmission came out of a 2002 C230, which makes it a later version with needle bearings but it still has the earlier style double-sided friction discs. Some of these friction stacks were changed to a single-sided friction style, the benefit of which I can only assume to be getting more frictions into the same space. I've sourced OEM rebuild parts (some German, some not) from Natpro for $199 as of 12/2014. Their kits come with the Mercedes star on each part, straight from the manufacturer! A highly reputable (and expensive) local specialty shop gets their kits here too. Seems solid to me.
So far, the master overhaul kit looks like basic gaskets and seals while the frictions are offered separately as a set or individually. The number of frictions vary from one application to another and the kit seems to come with enough for the basic setup. I plan to add as many frictions as possible in each stack, so I will have to order additional ones. The way it works is: each friction surface absorbs a certain amount of energy during engagement. The more frictions you have, the less energy absorption per-friction there is. This also translates into slower wear per friction. You can really appreciate the benefits of synthetic ATF by studying the innards of a transmission - it's all about friction!
Since I see no obstacle in doing so, I'm switching from double-sided to single-sided frictions wherever possible. Some stacks only come double-sided. At the end of each stack there is a piston used to apply pressure when the gear engages. The piston comes in at least two different heights, and I've heard of people machining them down to free up more space for additional frictions. I'm not sure that I'll be going that far but it is secrets like this that open up great possibilities for extreme torque.
One essential detail I should bring up is about friction stack gap. If you imagine a stack of friction discs (and steels if double-sided frictions are used) sitting in a cylinder with the apply piston at the bottom and a snap ring at the top, there's one thing missing: the gap. There MUST be space in there so things don't just burn up when free-wheeling. The necessary gap is up for debate. Apparently the factory has some fancy & complicated way to determine this, usually resulting in a pretty generous gap. The ATSG manual recommends a gap range of 0.25-0.33 mm per double-sided friction, or 0.13-0.16 mm for single sided. I've heard of about half that clearance being used in performance builds where longevity is of less of a concern. Personally, I intend to set up on the tighter side, perhaps just below the ATSG window.
At this point you may be asking how the gap can be set. For a single-sided stack, it's a matter of the number of frictions, and the thickness of the snap ring that holds it all together. For double-sided stacks, the thickness of the steels will be another variable - any steel in the stack can have a different thickness from the rest. If they're all thin, you'll get space for more frictions and or gap. The apply piston can also be changed or machined, which will affect friction space and gap too.
There's still more I could write about but if you've made it this far I'll have mercy on you until the next post. I think once you understand what to do, upgrading this transmission should be a breeze. Making it set up for the absolute maximum possible torque would involve buying lots of parts from AMG or V12 versions of the trans, which would get expensive fast. A basic rebuild and substantial upgrade seems easily within the reach of an adventurous DIY-er with way too much time on his hands.