None of the canon trials have any sort of "lawyer" as we would recognise them. Generally the wizangamot seems to fill the role of lawyer and jury in these trials, with Dumbledore being the closest thing to 'legal representation' at Harry's trial in OOTP.
As for your second question - while the owners of a nationwide law firm would naturally have made a decent amount of money, I'm not sure they would exist in the wizarding world or fit the setting of Harry Potter. I personally can't think of any situation where any of my characters would consult a lawyer, and given that the Ministry seem to conduct weddings I would imagine they deal with divorce as well.