I think you'll want to check out hook_user() to get it done... and what you've described sounds reasonable... check the author of the node, see if their profile has a value for this, and if not default to the store's address. You can use hook_form_alter() on the form with ID uc_paypal_wps_form to change the value of the hidden 'business' element to be the right e-mail address. (I'm assuming you're using Website Payments Standard here.)