Is there an NPC someplace in Fallout 3, or the expansions, who can repair the unique items (Chinese stealth armor, etc) to 100%?
According to the Fallout 3 Wiki -- Somah (An NPC from the Mothership Zeta DLC) has a repair skill of 100.
You may also look into Haley (An NPC from the Point Lookout DLC) who has a glitch that will eventually bring his repair skill up to 100.
If you are just looking to repair your unique items up to 100%, look into Alien Epoxy, a semi-repeatably obtainable consumable that instantly restores between 15% and 30% condition to an item. (Also from the Mothership Zeta DLC)
Just letting you know, Chinese Stealth Armor can be repaired with Recon Armor. However, you will need to use the aforementioned techniques to repair the Gauss Rifle. In addition, I am pretty sure that the Winter Armor does not degrade (which is honestly a little odd).