Hi,
Lot's of questions, I'll do my best to answer them.
1) I ordered at a firm called Topton (which I believe is Eglobal underwater). I have the i7 8850H, with BIOS v0.6 I believe. Not sure which machines have issues, some of the 9th series intel seam to be a bit different. In short, I have the normal S200.
2) Topton, choose a 8th gen machine. But there are some successes of 9th gen machines (9300h).
3) Search for those that are genuine compatible to a Mac. I have a ADATA XPG SX8200 Pro, but also the Samsung Evo's and Pro's work, although the 9700 Evo Plus has some firmware issue but fixable with updating. Ram same story, I have kingston hyperx 16gb in dual channel.
4) I think so, I use the m2 wifi port and use the required broadcom kexts. Everything is working. I am using the DW1560 m.2 card (bcm94352z / dw1560)
5) Both work, I invested in the switch to opencore because it is more future proof.
6) The only thing you can enable is Intel VMX (same?), you can enable that one.
7) yes, but this thread is really supportive to get you through. I shared my EFI folder for Opencore and others did for Clover so you can choose and adjust/try for yourself.
Good luck.