Yeah the sludge does rise to the top of the expansion, and I think I just about got it all out. I started by doing a complete cooling flush. Then every other day or so I'd remove the expansion tank and empty out the sludge/antifreeze until after about a week of doing this, it appears to be all gone, save a few drops.
The leak is very slow, because my oil level has not lowered noticeably, nor has much sludge returned.
I'm only guessing its the head gasket ... would seem the likely cause. As chowpit said in another thread, the #6 cylinder has an area where gasket separates the head from the coolant flow and is the likely spot of leak.
BTW, the head was cracked about 5 years ago, and was replaced for roughly $5,500 by a MB dealership here in Houston.
I hope to god the head hasn't cracked again.