justice League Cartoon Question

[Spitfire]

I've been on somewhat of a Batman kick lately and decided to buy into the Justice League series. So far the only set I have is "Justice League: Unlimited Season 1". Now from what I am to understand this is the third season of the series and the first in a re-vamp of the first show. Now from what I saw there's only four seasons and several movies.

So why I posted is to ask anyone who follows this exactly what DVD releases I should get. I don't want any movies that are just two-parters mashed together, which DC has done before(The first Batman Beyond movie anyone?). I also saw a movie called "New Frontier" that is supposed to be about them getting together is that tied into the series? I'm only one disk in and there's been several episodes involving past events that I'd like to better understand.

So if anyone doesn't mind could you please make me a list of the essential DVDs, including movies, to complete this collection. Also since it makes several references to Batman I wonder is this series also directly connected to the Superman series? If it is I might pick up that series as well.