They won't try suing you unless they actually have grounds. What you want is more than just the "balance" from the original creditor.
Below is a link that may help. As you'll read, they can't contact you more than 3 times a week.
What you need to do is send the collection agency a registered letter sayign you want no further contact unless they can verify the debt, and even then only via registered mail.
If they do for some reason goto court, they'll have to justify every penny of interest. Additionally, Small Claims has a "settlement conference" that is mandatory as soon as you file your defence, at which point (if it gets to court even) the collection agency will likely settle for 40-50%, on a payment plan.