Now, which parts should someone get in order to turn the new Catapult into a C3 model?
This is, to me, the downside of the approach taken.
You'll need to buy an OP-110 XXL Launcher Plates (has the Arrow IV launcher, and an OP-112 XXL IS Launcher pack. As long as you don't mind having assymetrical pods. If you do, you need to buy two OP-112s.
If you want a Catapult K2, you need to buy two OP-102 IS PPC packs, because there's only one of each size in a pack, and you need two. Plus you either need existing bitz to build the boxes the PPCs sit on (I'd suggest the XL packs in OPP-112), or face them on the existing missile pods.
I kitbash & customise extensively. I'd much rather have had each pack be 1 thing - eg. three Arrow-IV plates, two XXL IS pods, four standard IS PPCs, four Clan Medium Pulse LAsers. Yes, you'll rapidly build a bitz box, as you continually overbuy to get those two bits you need and all the others you had to get.
Still, IWM's putting their dollars on the line to release these, and they're welcome. For my two C-bills, though, it could have been better organised.