Well WHS2011 is cheap US$50 so it meets that requirement. It will also meet your needs for the numbers of Clients (up to 10). The only problem I can see is that your diagnostic s/w whatever it is may not be compatible. Loading images should be fine if you
are just using the server for storing the images in shared folders.
You are not using much of WHS2011's functionality (media streaming, client backups etc.) so if your diagnostic s/w is not compatible you may be better off with W7 albeit more expensive.
Phil P.S. If you find my comment helpful or if it answers your question, please mark it as such.