Further information about a collection agency and questions

I received a response to my inquiry to an attorney in Fresno regarding a debt on which they are trying to collect. They sent me the bill that indicated the account was charged off in September, 2003. They also sent me a Bill of Sales indicating the collection agency had bought the account in March, 2015.

I got these documents from the attorney who has been mainly dealing with my mother’s estate (and I got a summons during that time). The only other attorney I have is also a family friend (as is the first). Neither attorney seems to know much about this stuff.

Anyway, I’d like to ask if I’m relieved from the debt due to the Statute of Limitations (which I believe is 4 years here in California). From some other searching, it appears this will stay on my credit report until at least 2010.

Finally, is the charge-off bill (with my name and address) sufficient proof that I owe the debt? I would think they would have to present signed contracts or something of that nature.

Thanks to everyone and hope you had a great holidays.