Can anyone suggest some nice retextures for imperial buildings? Particulary the interiors?

My suggestion is to look on https://sites.google.com/site/tespages/Home/retexturelinks page for the word "Imperial". All the ones I know of are listed there. There are lots of linked screenshots to help your decision. If you have trouble finding any of them, you may find them at http://mw.modhistory.com/home or ask here or in the Mod Detectives thread.

Ah, many thanks to you! I'll try my luck there.