Good call. And if viewNX can't open the raw file, you might be able to extract the embedded jpeg preview from the file using something like erawp: http://www.fsoft.it/ERawP/
(no personal experience with that tool, but it was recommended on dpreview)
My ballpark figure would be 100-150W idle for the dual xeon setup. Taking out one CPU would probably drop you 30-40W.
Modern Ryzen still isn't very efficient at idle (at least not for me).
If you're really concerned about power cost: get a used SFF business PC like a Prodesk G4 or something. That'll get you idle draw around 10-15W (and chances are the CPU actually beats the Xeon at most tasks).