TUV as notified body usally requires extensive tests and then they don't need to come witness the tests every time when you have product fabricated. You could choose different notified body and you might not need to have extensive tests at all. But the notified body different from TUV might need to come to witness the vessel every time when the vessel is done. It is up to you really. It will be different from mass production and one time deal.
Also, TUV usally generate a comprenhensive list for particular material assessment tests for the material you use.
In addition, TUV as notified body require the both of PED and AD.
What I said about HP0 is really related to the notified body when you choose TUV as notified body.
Please let me know if your notified body doesn't need to have HP0. I would consider to switch our notified body to yours seriously.