EVE is basically spreadsheets in space and point and click combat, and requires a monthly sub. Some people really like it, but I could never get into it. The community is really interesting though.
Star Citizen and Elite are buy once play forever.
Elite was going to have single player offline, but they scrapped it because it wasn't technically feasible to do.
Star Citizen will have private servers and offline single player, the scope of these modes isn't really known yet AFAIK. The single player campaign will be offline or online with co-op, and the campaign will affect the persistent universe. I'm not entirely sure if there will be a single player offline universe. If you run a private server though, you could do whatever you want with it.
Elite is alot of fun right now, it's pretty amazing exploring the Milky Way. Frontier did a really great job with everything.
Star Citizen is pretty barebones right now, but I'm really pleased with the way it's shaping up. I've been a backer for a year and a bit now. It's been a lot of fun watching the development progress.
Star Citizen is also much more ambitious than Elite, it really gets my imagination going. I think if they pull it off it will easily be one of the greatest games ever made.
The combo of piloting, FPS, and life sim should be really great. They seem to be taking everything really in-depth and detailed.
Have you played Freelancer? Strike Suit Zero is fun if you just want to do some shooting. I would also check out Wing Commander Saga, it's free. As well as Freespace 1/2 if you haven't already.
You could also give X3 a try too.