This is Grump Mike's CNC mill project
http://www.thebox.myzen.co.uk/Hardware/CNC_Conversion.html
It covers a lot more than you need, but will get you thinking and answer a lot of questions.
You talk about monetary switches to control up and down movement, will you not also need limit switches to stop when you have fully retracted or descended?
It sounds like really what you are building is still mainly a manually operated drill, is that right?
You would turn the pot up for a fast descent, press the descend button, turn the pot down just before the cutting starts, hit the retract button when the hole is drilled, the drill would stop when it hits the full retract limit switch.
You will have a very smooth descent during the cut but the actual rate may vary depending on how easy it is to set the pot.