My opinion:
From what you said (single player, combat , aliens, trading) I would heartily recommend the X series.
I've mostly played X3 Terran Conflict (X3TC) the most recent one.
STEAM recently put on sale the X megabox (It's available elsewhere as well) - basically, the developers and the community who play the X games have an extremely strong relationship, and the new release is ALL of the X games ever released, plus all patches etc, plus an enormous wodge of content generated by the community.
It's a fantastic game, and you can play it a million ways. Check the official forums for it - a few people make storytelling posts, usually known as DID (Dead Is Dead) posts, where they play a game until their character is killed (often this takes months) and write up their onscreen exploits in a fun story-based way.
It really is an almost infinitely replayable game - one run through you might spend nearly all your time in a small fighter craft blowing up aliens, the next you might play as the boss of a mighty trading empire with (literally) hundreds of ships under your command, plus space stations, factories and even your own shipyards. One of the best time-and-enjoyment-vs-cost investments I've ever made.