Try these people as a baseline (i'm not recommending them or vouching for them as I have no experience of dealing with them but the site implies that they know what they are up to. You also need to compare notes with other MB owners and get a general consensus about modifications and tuners - try posting in our Mercedes forum and I'm sure a few guys will pop up and chat with you!)
http://www.mercedesperformance.co.uk...erformance.pdf Most other prices are round these marks. NoS can be fitted for around £500.
You really want to go for a custom remap and explain your requirements to the tuner.