Morning Stevellew, I have a fault finder i brought from Ebay mine was only about £20 quid it is a D900 version 2013. the numbers on the front are EOBD/ OBD11 live data, i have not used it on my MB yet but have used it on my previous Ford and found that it worked well, CD included with error codes, it helped me find Vacume faults on my Ford and as it works on all vehicals i assume it will work just as well on MB's
As said above, these seem to work basically on Engine faults or when lights appear on the dash. I50 quid seems a lot to me, probably a good investment if your in the business, but mine has only been used a couple of times, and has been very helpful.