This is kind of one of those 'how long is a piece of string' questions, I think, because it's really up to you. The main question is how much of a 'slave' to the canon setting do you want to be and how much of a stickler for fine detail are you and your friends? If you're the GM, how much do you like 'writing' the setting as opposed to reading it from a book? You can run a perfectly fine game using just the setting information in the core book. It has enough rules and the essential setting information to have a game work basically anywhere except maybe the Jovian Republic, which only mostly becomes a semi-plausible society if you have Rimward. Sunward and Rimward just expand on what's already there for the most part. I've not gotten around to reading Panopticon yet but so far what I've found is Sunward and Rimward are both really good - they've come in useful for the game I'm currently running which went from the Main Belt (Rimward) to Mars (Sunward) over the last few sessions. They have tons of setting information that you'll probably find useful. I probably wouldn't have needed either if I had more free time to write the sessions, but since I'm working full-time as well as being a GM, having the books to help was definitely welcome. Gatecrashing is something I'm still working through and it's quite different from the other books in terms of structure because of how specialized it is in comparison to a book like Sunward which covers basically everything outside of the Inner System. You'd only need Gatecrashing (and again, need is a relative term here) if your game was centered around gatecrashing or at least was planned to heavily feature it. Otherwise, it's not going to be at all important, at least from what I've read of it so far. I'm kind of with you in that I can take or leave exoplanets for the most part. Anyway, that's my take on it.