I am finding that my own personal definition of collectible is evolving. I started out mostly in the camp G.T. describes, +85% original finish and all matching parts. Then I discovered in practice, the acid test of collectible or not for me personally is, am I willing to shoot it? I have found I don't like to shoot all matching pistols with a quality refinish, or pistols with 60% or better original finish that have all or mostly matching small parts.
I'm also finding that for collectibles, I prefer pistols with visible wear over pristine examples.
Interestingly, this link defines 'collectible' as matching with 60% or better original finish remaining. Go to the 'general information' tab, open the FAQ link and page down to find the description of "collectible"
http://www.lugerforum.com/