Whats your budget?
Does it absolutely have to be 12s?
Sensah and Ltwoo have 12s options on Aliexpress but they have very mixed reviews on youtube. To my knowledge, there is no chinese derailleur with a clutch. I wouldn't ride without one, but it may not be a big deal for you.
If it doesn't have to be 12s, Microshift Advent X is a good affordable option. I think I'd go with Sram SX before going with a chinese groupset for mtb.
As far as brakes go, you can find super cheap mt200 on Ali and eBay. Meroca also seem to have a good reputation, but I have no personal experience.