With a work envelope of 2x4x8 an X2 mill would be big enough. They can be bought new for about $500. If you can find an RF30 or larger mill for a decent price, even better. There are a number of threads that discuss CNCing these mills.
Since you haven't machined anything before start with some manual milling.
That will give you a better idea of what you are getting yourself into.
If you really want to build your own from scratch, take a look at
www.cnczone.com/forums/showthread.php?t=21872
this was designed to route wood but apparently is also capable of light cuts in aluminum.
JohnW