Great game -- if you haven't played it before it's well worth taking a look for inspiration or just to see how they approached certain problems. There's a free open-sourced version available under the title "The Ur-Quan Masters", based upon the source from the original game which was released to the public.
Having better ships make new planets available sounds like a key and lock system, where perhaps certain planets have atmospheres that are destructive to ship alloys, or some ships need certain kinds of magnetic shields to be compatible with the magnetic fields of the planets.
Why is it upgrades for a ship that gets you further.
Even on one planet it's a PRETTY BIG DEAL moving from country to country, sure you can go plenty of places but not just anywhere or at least being able to do something useful anywhere on the planet. To make things worse your character has a spaceship, that's pretty much like saying "Hi I'm moving from country to country but I'm bringing several thousand people with, is probably the "mega-artillery of the future", and probably follows robinson's/jon's laws* ". Now you get to scarier things like import/export laws, bacteria/viral infections, taxes, paperwork, and racism...
Here's a nice website to consider if you ever want to research a space game, it's pretty disappointing more developers don't use it: http://www.projectrho.com/public_html/rocket/
*Robinson's - Anything reaching 3KM(2Mi)/second packs it's own weight in TNT of explosive power on impact Jon's- Any sufficiently interesting form or propulsion is a weapon of mass destruction(IE cut other ships in half, move space stations out of orbit, set atmospheres on fire, and similar without remotely trying).
