For the turntable, I have found a complete direct drive "pro" turntable it has 5kg of torque which beats anything else available these days. Technics 1210s have been used successfully but at 1.5kg torque wow/flutter does manifest itself. What i have found should suit my purpose, but if i get to the stage of building my own i would look at using an precision lapped thrust bearing, oil bearings/drives have been used but they prob. too complicated or expensive for my purpose. The use of heavy platter reduces wow/flutter too. Whatever - i will be looking at pro turntable designs old and new, i don't feel it's necessary at this time to personally further development in this field! Most of the lathe parts should be off the shelf.

Cutting heads - There are a couple available, i intend to build my own, there have been successfull heads made from a pair of tweeter (speakers)

On the feedscrew again, i would prefer as many off the shelf parts as possible including the nut, i am now going to look again at what's available

Neumann lathes start at around £10 000 these days for a VMS66 upto maybe £45 000 for a VMS80 (DMM lathe) setup. It's extremely unlikely that any studio would put a lathe in the skip, they're too rare and valuable. An pro audio broker would bite your hand off, the last few i've seen were sold rapidly!

I haven't seen a lathe driven by gravity! But i know one of the first portable disc recorders (during WW2) had a turntable driven by a wound spring like a gramaphone, and a battery operated electrical cutting head, they were used in the trenches!