While I agree on the errata, I disagree on involving the other system books, especially when starting out and when looking for things to spice up your games. The systems don't mesh together all too well right now, especially Black Crusade..
Back to the main questions:
1. This is the core book you will be needing to play:
http://store.fantasyflightgames.com/productdetails.cfm?SKU=RT01 It contains everything you need: eight player classes, full rules on playing the game, a handy (and must read) chapter on being the GM and chapters tackling the following 40k lore: the Imperium, Rogue Traders and the Koronus Expanse, the provided sector for you to play in. Finally there's a chapter on some of the enemies you can throw at your players and a complete three act adventure.
2. No. All your players really need a book for is when they need to spend their XP on new skills and talents or when they want a shiny new toy to play with. Though as Blodwynn has mentioned, handing out PDF's of the main book might be a handy way of priming your players for the games.
3. Pretty well actually, if the players or GM take the time to read the provided background to get the gist of the setting and just how fancy, important and special their characters are when compared to the rest of the Imperium.
4. I suck at this, but here goes. The game uses a D100 system requiring just a few D10's to play. Say your character has a BS of 37 and wants to shoot a mutie at normal range. This means you need to roll a 37 or less to hit the mutie, ignoring the +/- modifiers right now. As with most other RPG's though, you can make it as dice heavy or lite as you want to. I don't let my players roll for everything, not when they can act a conversation out or try and seduce the GM.
wink Here's some other fancy stuff that you might like:
The Forsaken Bounty, a fun and free demo adventure for the game, comes with all the rules you need, plus several pre-made characters: Rogue Trader, Arch-Militant and Seneschal. There's also an expansion document with two extra characters, the Missionary and Void-Master, which can be found
here. If you're still not sure about the game, the demo adventure has a sequel, again free, called
Dark Frontier that perfectly ties into the first demo adventure and with a little tweaking can be used as a jump start for the three act adventure found in the main book or as a future seed for another adventure.
Need more fluff? Here's "
Secrets of the Expanse" and
Epoch Koronus.
Need help building ships? Try
Dry Dock, their primer document on how to build ships.
Finally, concerning future purchases. If the first few sessions go over well and you've got a taste for more, then I highly recommend you get the following books:
LURE OF THE EXPANSE, a massive three act adventure that can handily be broken up into at least a dozen sessions, maybe even more when your players get to Act II. My player group had the most fun with this series of adventures, which plays out like an intergalactic treasure hunt between you and quite a few other rival Rogue Trader families.
INTO THE STORM, a must have when it comes to equipment, alternate careers and for those inclined, Xeno crew members (Ork and Kroot). Not just merely an equipment book, it also offers alternate origin options, alternate careers for your PC's, rules for using vehicles other than massive starships, enhanced game mechanics for those looking for challenges and a nice chunky bit of exposition on Port Maw. Again, my own player group had a blast going through the options, two of them opting for alternate careers when they reach the proper ranking.
Hope this helps and remember: Ambition Knows No Bounds.