You need only provide what they ask for. However, in an interview setting, some questions might lead to other questions. You can either be prepared with documentation or use delaying tactics by responding with a request to reschedule to give you time to get the documents. Personally, if I were in your position, I'd make the vultures work as hard as possible, knowing I'd have BK available to me and that I'd never have to pay them.
If you decide to be prepared with documents, your loan contract and current balance statement would probably suffice.
You needn't pay for anything. This is the creditor's ballgame. The creditor pays for any services. If the creditor's assessment of your home value comes up a lot higher that you think the home is worth, so what. Instead of spending $400 to $500 on an appraisal to challenge that, you'd be better off applying your money to bankruptcy and cut the creditors off at the knees.
Barring that, you can look up your house on Zillow (try it) and get an estimate of the market value and compare it to sales in the neighborhood (also shown on Zillow).
You wrote that the subpoena duces tecum came from the $320 creditor. I seriously doubt that the creditor will spend too much money on collection efforts.
If the subpoena doesn't involve your appearance for an interview and just requires documents, I suggest providing only the exact documents required by the subpoena, no more no less. Then wait and see what happens.

