Need help to budget out a 12600k based gaming pc

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Hello all, I'm from the USA and I need help budgeting out 2 12600k based gaming PC for my friends. The budget I'm trying to work at is $400 USDM or, if it's even possible, even cheaper. I already have 2 12600k CPUs and 1 pair of 16gb ram. I also have 2 1tb m.2 sticks and 2 1tb hard disk drives. I think I'll go with a 1080ti unless you guys recommend something different. I'd like to preface this part.I would profess to buy all the following parts on Amazon or at least one website if possible. All I need now is some recommendations for a budget pair of 16gb DDR4 ram, motherboard, power supply and maybe a cooler if an oem style cooler won't work. Not a must , but a cheap pc case with fans and good airflow. Thanks for the help, and hopefully I don't come off as too naive or stupid.