I was in a similar situation as you and thought about using Lexington Law but decided against it. They do things that you can do yourself but charge a shitload of money for it. Go to MyFico.com and read up on their "Rebuilding Your Credit" forum. Even post about your problems and people will chime in with excellent advice. You will learn about "goodwill letters" and "pay for deletes" which can help your credit greatly. They also have a lot of information about how long derogatory marks stay on your credit report. Basically, though, if these marks are over 7 years old, they should have been removed already. They can still send you collection letters and sue you until the statute of limitations is up but they MUST remove the marks after 7 years or they're in violation of the Fair Trade Agreement. The advice from there has helped me tremendously and I'm in MUCH better shape now than I was 2 years ago. I have obtained a new mortgage, got good rates on auto loans, personal loans and credit cards. Good luck.
Everything he just said was 100% completely spot on. MyFico credit forum is the tits.
It's best to read up and learn about credit and then try to dispute/rebuild yourself. As mentioned earlier in thread, you can do everything yourself what Lexington will do for you...except they are getting paid no matter what and you are looking for true results even though no results can be guaranteed by a repair company.