I visited Strausburg several weeks ago and got to see the new tender for 346. It is all stainless for the cistern and the frame was rebuilt with new wood sills and decking, but all the original hardware was cleaned, repaired and will be re-installed.
The tank will have machined rivets welded on. These will be exact duplicates of the originals, right down to the dimples.
The explanation for the stainless (which will also be used on 20) is that it was cost effective and will last a loooong time.
They have also duplicated all patches but not the dents.
They have already cut the hand rails off of the old tank (crudely) and I guess the hatch cover and other things will be used in the rebuild.