Hi Mark, welcome to the board! Before I attempt to answer your questions, I would like to preface that these are my own opinions (that of a beginner) and you should wait for others to chime in.
Answer 1:
You will find differing opinions on this matter. I will let the more experienced members of the board answer this one.
Answer 2:
The WW2 mounts/koshirae can be referred to as gunto koshirae and there are several variants. It is fine to mount gunto in traditional koshirae. For long term storage it would be best to leave them in shirasaya. The latter provides better protection from the elements.
Where do you usually look for nihonto? You should be able to find gunto in their original mountings within the USD3k-5k range with no issues. This forum has quite a number of links to commercial sites. Under the drop down menu, go to Nihonto Info > Links.