The first thing I suggest you do before engaging a lawyer / consultant is to assess your eligibility and get an idea of your CRS score. If your score comes to less than 400 points, I won't engage a consultant as that's money not well spent. You have to first be eligible based on your score - age, education, English proficiency etc. No one with less than 430 crs points has been issued an invitation. You can calculate this score yourself and come up with a good estimate (see link below). Once you think I have enough points to even get invited to apply, then you can engage a consultant to help you deal with the over-stay complexity. If you cannot get enough points to get invitation to apply, there's no point getting a lawyer.
To check your chances of getting an invitation use this tool directly from CIC website, answer to the best of your ability. If you score approximately 400 points, get your lawyer to start figuring how to guide through if you get an invitation with your overstay issue.
http://www.cic.gc.ca/english/immigrate/skilled/crs-tool.asp