Financial data on any client is confidential. Although, mortgage and deed information is readily available - current balance data is not. If an agent were to come into possession of this information they would not share it with anyone.
I'm sure you have read this clause before, but I think it bears repeating. Agents/Brokers are held to a very high standard of conduct. Information you need to make a decision will be given to you - no question.
However, balances etc. have no place in determining price you as a buyer would pay. Comparable Sales, Current Market Value/Activity, and individual buyer's desireability are what come into play.
I hope this will dissuade you from pursuing this area; as in essence you are asking people in the know to divulge information that is confidential.
Unless you assumed the seller's mortgages, I don't think it's any of your business.