The only real investment is Basic Set: Characters and if you also plan on GM'ing then you need Basic Set: Campaigns. Beyond that the other 4e material is "nice" but not essential as their 3e counterpart in most cases is still relevent or can be used with slight modification.
But all that aside, if you are interested in this game, then you don't need to invest in anything to play. The GURPS Fantasy Grounds ruleset is free, and you can make your character using GURPSLite and the templates I have provided in the character info document.
If you want to join us in our adventure into the Interstellar Wars, then hop aboard.
