You want to deal with the entity that owns the account or has authority to settle account on owners behalf. be careful who you are dealing and make sure you get any settlement in writing. Might also want to check your credit report to see what entity is reporting the debt to the credit bureaus.
Paying off old debts will not help your credit rating very much, the damage has been done. The original creditor has charged off the debt, they cannot accept payment once this has happened. The collection agency will gladly take your payment since they only paid three or four cents on the dollar when you pay them in full their wallet will be fat.
Filing for bankruptcy could wipe all of your debt, give you a fresh start, and let you make payments on cars or homes which will help your credit more than anything.
I am not your attorney unless you and I have signed a retainer agreement. What I am saying is not legal advice. Do not act on this information without engaging my services, this is for consideration only.
Paying a debt collector to settle will usually be much less expensive than paying the original creditor to settle, but paying less than the full amount owed is always bad for your credit since settling requires that the debt be deemed less than fully collectable. You need to decide the cash value of a few points either way on your credit score & act accordingly. Hope this perspective helps!
Deal with the original creditor if they still own the debt and if they will talk with you. If they still own the right to collect (haven't sold the account off to a debt buyer) and insist that you deal with their collection agency, then you will want to get any settlement agreement in WRITING before sending any payment whatsoever. Do NOT trust their word. Do NOT let them have access to your banking account or credit card. Any payment should be by cashier's check or money order. Whether or not paying off the debt will help your credit improve in another matter altogether. Please meet with a local consumer or bankruptcy attorney as soon as possible. Good Luck!