I'll try to answer a few of your queries:
If you want an AMG 6.0 motor, you'll need to talk to (according to the AMG Brand Mgr at MBUSA) Barry Taylor at AMG West. But be advised-few on this board, if anyone, trusts the guy
You can also try
www.performances.com , which is AMG Canada.
As to a rebuild, I think the biggest problem with increasing the displacement of the M119 motor is with the silicon-impregnated cylinder walls...they are not sleeved. I really don't know how they bore out these blocks and re-do the cylinder walls, but that's got to be a huge chunk of the astronomical prices the folks like Brabus and RennTech charge. I don't think anyone sells a kit for user-installation.
The engine was very successfully turbocharged by Mercedes, who campaigned this motor at LeMans, et. al. in the early 80s. I am not aware of any aftermarket turbo setups, but there are a few blower setups available.