is it necessary to have a lawyer for an ip when dealing with a well known invention company

You have to be careful when dealing with an invention company, many, even well know ones, are unscrupolus. Check the Patent Office website for information on the subject: http://www.uspto.gov/inventors/scam_prevention/#heading-1 You may even find it less costly to use an attorney rather than an invention company; you could get more for less based on what I have seen such companies charge.  One thing you should ask the company is: "what percentage of your clients have recovered more money as a result your marketing efforts than they spent in fees to you". I would guess that it is a very small persentage.