Clickykeyboards is a great place to buy used and new in box, never used, original Model M boards. (New in box models can fetch upwards of $200!) I bought a used M from them and it was spotless and works great. I do prefer my Unicomp in general, though. It's hard to explain why. It might be the newness of it, but it feels a little tighter and crisper. The differences are subtle, though. I will say that the Unicomp has a more "gravelly" sound while the M I have is a little springy. You'll find differences from board to board. I know some people who have five Ms and one or two are their favorites while others end up in the closet for various reasons and never get used much.
If I find myself in a position where I'd need to buy a new buckling spring board, I think I'll just get another Unicomp and support the small company.
Take a listen to this:
http://www.npr.org/templates/s....php?storyId=100076874
Click on the "listen" link. That story solidifed my desire to abandon cheap rubber dome boards and commit to replacing all my boards with buckling spring boards. It makes writing much more pleasurable and gratifying. It also leads to more accuracy due to the significantly superior key response.
You may also want to consider a board based on white alps switches. There are cherry switches that you might find satisfying as well, though some of the boards that use them are available only in Japan and require hefty importing costs or were only made for a brief period of time before being phased out. I'm not a fan of the white alps because I feel they're too noisy compared to buckling spring and they response isn't nearly as purposeful feeling. You really just need to try different switches and find out what you prefer.