GURPS Traveller (which is a GURPS 3e product) and GURPS Lite for 3rd edition make a pretty good set, if you're okay with playing in 3rd edition. I've run several campaigns with those as my standard.
For GURPS 4th edition, I recommend
GURPS Traveller: Interstellar Wars, GURPS Lite (4e), and
GURPS Ultra-tech: Weapons Tables.
GURPS Lite for 4th edition has a very skimpy list of modern firearms, and no ultra-tech weapons (lasers, etc.) at all. The Weapons Tables file is a cost-effective alternate way to get more options; otherwise, you'll need Basic Set or Ultratech. Interstellar Wars has both ship design and world-building rules, so you won't need GURPS Spaceships or Space to start.
Also note that the entire GURPS Traveller catalog is available digitally from
Far Future Enterprises (Marc Miller's company), now that SJ Games' license has expired. It comes on three CD-ROMs for $35 each. If you order all three, Mr. Miller will refund the cost of a fourth disk (from one of the other versions of Traveller).