Hope I did not come off as being rude.
The accepted best place to get the cables is Lonelec in England, I have a set from them. Order online and about a week latter the cables show up. The ones on the rain forest I also have a set of. Both work, I would trust the Lonelec more than the others.
Guzzidiag is pretty straight forward. There is at least one lengthy tutorial on how to use it here. Many shorter threads on specifics about it.
I guessing you want to look into a Quota. Your service light "should" be able to be cleared?? Check for error codes, fix the issues. Just as a thought, this has been brought up before, I always forget about this easy fix. Disconnect the battery overnight, sometimes that can solve ECU issues. Also some say to disconnect the battery cables from the battery, then hook them together can also help.
Tom