Machine construction.

The "what is accurate" thread got me thinking about machines I would
like to build and how design influences the potential accuracy of a
machine. I would like to build a small mill for 3d work.

What is the most inherently accurate machine configuration?

How can a hobbiest on a budget get the most machine for his money?
(Best guides, screws, building material, etc.) (For this question
let's assume a 18" X 12" X 12" work area and a 3k budget.)

Rob.