Hello Torsten,
1. My bad, the best solution will be to directly use the "Attached Files" option that you can find via "Hikashop->System->Emails->' Order status notification' ".
However, these files will be sent after every status update.
2. If you need a second "required" checkbox, the solution will be to directly create and use an "Order" checkbox custom field that you'll be able to create via "Hikashop->Display->Custom fields"
4. Sure but using custom fields will only affect the "Fields" step of your checkout workflow, if you want to change the displaying of the term and condition step, you'll have to directly edit the code of the "show_block_terms" file of the "Checkout" view of your front-end template via "Hikashop->Display->Views".
Best regards,
Mohamed Thelji.