the majority of the games i play are single player. note there is not a campaign but it still has high reply value. you should give the game a shot. the demo is available on impulse. i havent got into diplomacy much yet, but entrenchment is great and the original version is great. lan games are a blast too.
note the the title is Sins of a Solar Empire.
Here is a link to the base game. You can get the demo here. If you are looking to buy, i suggest Trinity since it has all of the expansions...and of course buying direct from Impulse!
the expansions offer tons of new features. I rarely play the original game now. If you get tired of the game, there is a vibrant mod community as well. I know of StarTrek, Galactica, and other mods that are working for total conversion. there are others that simply enhance game play and graphics.