Midland Credit Managment ,MCM SOL out out on a charge off account The $400 bill is now $1000 Oaks Pennsylvania

I received a letter that MCM has now purchased a debt and wants it paid. They want $1,100. I checked the credit report and the original debt was charged off and sold 3 and half years ago. ( this is the 3rd company to purchase this debt) The SOL is 4 years in my state. The sold date was the last date on the account. The charge off date was 6 months before that date. I want to send them a letter of validation and tell them they can not contact me except through the us mail. I only have 3 weeks left on this 30 day notice. Like many Americans I am unemployed and no hope of getting a job. After reading about the bottom feeder attorneys on RipOff I am unsure of what to do!
Help! How is the best way to handle this letter and company?

S.
Jacksonville, Florida
U.S.A.