Couplers - Must have
Limit switches - You can get the machine running without them using software limits. Then you can use the CNC to make brackets if you want to.
E-Stop switches - Need to have one on the driver box at least, but could be added after basic machine is up and running
Control switches at the mill to stop/start/pause a job - Not strictly needed. Keyboard shortcuts on the PC do all of this just fine.
Cabling - Sure Needed
Enclosure - Not initially needed or really ever in reality. Another nicety that can be added later if wanted enough.
Spindle controller - Not a necessity. You can turn the spindle on/off just like a manual mill and still operate in CNC
Lubricant/Mist/Flood system - Very nice to have but you could use a squirt bottle from the dollar store and a hand oiler for a long time.
Tooling, you will want to look into TTS, something that is indexable. - This all depends on what you want to do. TTS is an option, but I prefer standard R8 tooling, and most of the time I use HSS end mills and very little indexable tooling even though I have them.
A pendant - Already discussed. Not a necessity, but rather a nice to have.
https://cnc4pc.com/c41s-pwm-variable...rol-board.html