The cockpit part is the headset, handlebar, stem and grips.
The missing thing are the seatpost, seatclamp, saddle.
Don't forget the rotors, thay are not always part of a brake set.
For the cockpit, seatpost, seatclamp and sadle, you can look on AliExpress. There are some cheap and descent kits (handlebar, stem, seatpost), less than 100 USD. I got my saddle also from AliExpress. You can find there a lot.