That is indeed the place where such an action could go. I’d do it like this:

   foreach ($ids_arr as $member_id) {
       $res = eme_member_set_paid($member_id);
       if ($res) {
               if ($send_mail) {
                       $member = eme_get_member($member_id);
                       $res2 = eme_email_member_booking($member,$action,$queue);
                       if (!$res2) $mails_ok=0;
               }
               if (has_action('eme_member_paid_action')) do_action('eme_member_paid_action',$member_id);
       } else {
               $action_ok=0;
       }
   }

Then, in your action, you can call eme_get_member to get the complete memberinfo for that memberid (and eme_get_answers for custom answers). There’s no field yet for passwords though, although adding a new fieldtype is very easy to do (adding it in the function eme_get_fieldtypes should be sufficient).
Make sure to link the newly created WP user to the person in EME (the person is linked to the memberships for that person) using the column wp_id in wp_eme_people. And for passwords in WP, use wp_set_password or wp_hash_password.

EME has already several only payment gateways integrated. But why would you still need another member plugin? I’m guessing writing a simple function that denies access to a page if not having a specific membership isn’t that difficult …