The kit comes with loose crossover parts, unless you order the assembled crossovers, then it comes all assembled, tested, and pretty. When they’re back in stock just order the 1099’s (there will be a box to check if you want your center to be horizontal), order three 1099 flat packs, and order 3 assembled crossovers. As for “parts needed”, check out a few 1099 build threads here. Everyone uses clamps (get a bunch of harbor freight 2’ clamps and a few 3’), everyone uses glue (tight bond is very popular), and everyone uses speaker wire (I used 14 ga, but I’ve seen everything from 12 to 18 used). Beyond that it’s personal preference for insulation, terminal posts/cup, and crimp terminals. I think I listed everything used in my build thread, and many others here did as well. Once you get started it’s pretty straight forward. If you get stuck just ask, lots of help in the DIY forum.
As for subs, the most popular seem to be the Marty and the vbss paired with Behringer amps. There’s a dedicated thread for each with listed parts, measurements, etc.
If you have specific questions ask away
SPEAKERS: 3 x DIYSG 1099's LCR; DIYSG HTM 6's and 8's surround; 4 x RSL C34E's atmos.
SUBS: 4 x UM18-22 Mini Marty's (and always building, currently working on VBSS's).
SIGNAL CHAIN: Denon X4400; Minidsp 2x4 HD; 2 x Jensen Iso Sub 2RX; 2 x NX6000d.