there is no such thing as a junkyard Mercedes, I've never seen one in my life, not even a mangled up wrecked one.

I would try ebay and look for used heads. I don't recommend the abrasive media porting, its doesn't really work, and its very expensive. You best bet is to find a machinist who uses a 5 axis mill and has a good history of porting heads on a cnc. Most likely he will know where to take out material and where not to remove material, this is the problem with the abrasive media, it removes all material equally through the port runners, which is WRONG! You don't want this because you will loose air flow velocity in certain sections, which might cause a bog or a lag time while it waits for velocity increase. This is why you need to find a professional who does it for a living and has cnc for consistency in cutting. You will end up paying for a lot of work, not worth the effort unless your going to sell 3.2 MBZ heads, and then you might as well keep going and cut larger valves with improved valve angles. Trust me you will end up mortgaging your house and your wife will leave you. lol.

The extrude hone is really only good for intake manifolds and maybe cast iron exhaust manifolds, it's not used for heads.

Your best bet is to supercharge the engine, its cheaper and easier, and there are more pro's out there who understand the technology better, so you will be up and running sooner with less cost to you. worst case scenario you will end up spending at least $5,000.