I don't have too much of a collection so far, just the core set, expansion TIE fighter and Falcon, but I was thinking of grabbing a couple of small expansions in tomorrow's (NZ) May 4th sale.
I was contemplating just grabbing the Y-Wing and B-Wing because they're personal favourites from Star Wars, but am I missing a trick with some of the other expansions? I've seen the TIE Interceptor mentioned, but I'm not that deep into the game that I know any better!
Every expansion has useful stuff in it, so get whatever you want. At least one of everything gives you every option. There are some expansions that synergize with each other, though, so you might want to buy certain groups as you build your collection..
Here's a post I made earlier in this thread talking about some of those groups:
Essential purchases to enhance your TIE Interceptor: Imperial Aces (for Push the Limit and the Royal Guard title, as well as more Interceptor pilots), StarViper (for Autothrusters) and either the M3-A Interceptor or Slave I (for Stealth Device).
Essential purchases to enhance your Millennium Falcon: Slave I or YT-2400 (for Gunner), Tantive IV (for C-3PO and R2-D2 crew) and some Z-95 Headhunters (great escorts for a YT-1300).
Essential purchases to enhance your TIE Bomber: wait for Wave 7 bombers to be released; also, Proximity Mines are only found in the Slave I and IG-2000. Other ordnance you don't have ... Ion Torpedoes (StarViper and Decimator), Flechette Torpedoes (E-Wing and Rebel Transport), Homing Missiles (A-Wing and Slave I), Ion Pulse Missiles (TIE Defender and Z-95 Headhunter) and Proton Rockets (Rebel Aces and YT-2400).
Essential purchases to enhance your TIE Advanced: wait for the Imperial Raider to be released
For your situation, Kryten, I would consider buying a second Core Set. It gets you three more ships for less than the cost of buying them individually, plus more dice (which you need) and a spare set of templates. And more TIE Fighters are useful.
Another thing to consider is an X-Wing expansion to gain access to Wedge (one of the best pilots in the game) and Garven.
If you buy a Y-Wing, it's fine on its own when you equip it with an Ion Cannon Turret and maybe an R2 Astromech or R2-D2. But there are two expansions that add things to the Y-Wing's game: the Most Wanted set, which has Bomb Loadout, Autoblaster Turret and the BTL-A4 title; and the Rebel Transport, which has R4-D6 and R3-A2. They both come with Flechette Torpedoes, too, which are cheap enough to be useful. (If you want to use the Bomb Loadout that comes with Most Wanted, you'll have to buy a ship that comes with bombs, like the Slave I or the TIE Bomber.) The other turret in the game, Blaster Turret, is underwhelming but if you want it you'll have to buy a HWK-290, which at least comes with the useful Recon Specialist crew and another copy of Ion Cannon Turret.
Keep in mind that while Most Wanted is a Scum faction box, its models and dials can be used for other factions. So buying it gets you another Y-Wing and two Z-95s, although to legally fly them, you still need the pilot cards and cardboard bases from the Rebel Y-Wing and Z-95 Headhunter expansions.
As for the B-Wing, it has obvious synergy with the Rebel Aces pack, which comes with new B-Wing pilots, the E2 title that enables the ship to have a crew upgrade, and a couple useful crew cards (Jan Ors and Kyle Katarn). A lot of people like running Tactician on the B-Wing, and that comes with the TIE Phantom. Possibly the best system upgrade for the B-Wing is Advanced Sensors, which can be found in the E-Wing (which also comes with a couple useful astromechs) and the Lambda Shuttle (which also comes with Heavy Laser Cannon, several useful crew cards and Sensor Jammer).