I bought a star system off ebay a while back and it work fine on my 03 atego with the big round port. It was around £700 if I remember right and includes a second hand laptop with Xentry diagnostic software, WIS and EPC installed, the multiplexer interface and a host of cables for Mercs of different ages and types.
Nick, thats as expensive as some that i have seen, does it work ok and access all areas?
It works 100%, and accesses everything, although you cannot change the speed limiter without an additional code. The laptop battery was basically dead on the laptop that came with my kit, but otherwise I am completely happy. It's pretty straightforward to use.

When I had a boost leak, the active fault page automatically linked me to the charge air system pages in the WIS. Pretty neat.