So here it is: the best Space Sim ever made. Yes, really. Freespace 2 contains some brilliant missions, fantastic mechanics and amazing spaceships - which is a fair trade for the lack of a sprawling persistent universe. Until space sims make their triumphant return (please?), this is a fantastic price point with guaranteed Windows compatibility and a £9 saving compared to Impulse Driven.
Note that the GoG page also links you to a modding portal, which will extend the experience indefinitely.