Arduino USB Pendent and Coolant controller

Hello,
I just started using LinuxCNC to drive my DIY CNC router, i've already done the basic configurations and all works just fine, i planned to build a pendent for my cnc, but, i don't know where to start from, any suggestions?

The pendent should have those features: axis jogging, home, feed hold button, cycle start button, ESTOP, an LCD i2c display, some buttons to trigger some macros like a custom probing routine, coolant ON/OFF buttons (i have in mind to put 2 types of coolant on my machine, flood and mist) and maybe some controll even on the spindle, for example an OFF/ON button with relative LED to show the status of the machine. Are those ideas possible?

Another question: Is possible to use another arduino as an output method for controlling relays for example?
Thanks to everyone that wants to help me.