Hello David,
Please pardon my rambling style. I'm typing and thinking "on the fly".
I recently acquired a Newing-Hall TLC-400 and am in the process of discarding the old "brain" and adding a true Z-axis, so I can probably offer you a bit of assistance.
You don't say what generation your NH600 is. If it's an original TLC, it will benefit from the lead screw nut upgrade offered by NH. Later machines also have cleaned up wiring and plumbing, a lot of which will go away with a Z-axis conversion, but the "chain" type cable guide is nice to retain.
You might want to keep one of the pneumatic circuits so you can use the original spindle block assemblyy for straight up "nose contact" engraving, where part of the the spindle actually rides on the work piece, thus maintaining a constant cut depth when working with irregular surface material.
I have not yet made any chips on my machine as it's a work in progress, but the stock X and Y steppers seem to be reasonably decent. Unless yours is a much newer machine than mine, replacing the steppers also necessitates installing new lead screws and nuts, as the older machines have integral lead screw-steppers. I certainly would not replace that stuff until I had determined that they needed to go.....
The biggest hurdle you face is the spindle itself. It is not practical to convert the original pneumatic actuated spindle block to full Z motion, so you will need to either make your own or spend the (ouch) $2400.00 that NH wants for their version. I am in the process of building a Z axis based on a THK ball screw slide with a Proxxon rotary tool. I will need to fabricate a mount for the NEMA 17 size Z-motor I'll use and one to mount the slide on the NH gantry.
I've settled on a Gecko G540 stepper driver which will be used in conjunction with a relatively low cost controller. The controller, a CNC USB Controller ( www.planet-cnc.com) has moderate performance, but only cost me $170.00 with software (I have great hopes....) on eBay. All of this has yet to be put together and tested, so don't run out and buy based on what I say here.
My NH400 came with a controller, but it was only semi functional and was only 2 axis anyway, so I scrapped the computer and driver parts of its innards and kept the (quite stout) case with power supply to house the new electronics.
I'm still just past the dismantling point. I won't start seeing the new components arrive for at least a week. Once I have all of the disparate parts in one location, I'll start the building process. I'll try to take some photos as I go along and post them.
If all goes well, and I'm pleased with how well the overall performance of the machine is, I will probably adapt a more powerful motor/spindle to my Z-Axis. I also have designed a ground-up drop in replacement spindle block that would use the NH belt drive system and the NH spindles. It will cost me about $1000.00 to assemble a prototype, using an outside CNC shop to make the parts. I have a manual mill but the parts are too intricate for me to want to make it that way. Consequently, I'm going to hold off until I see how the "first step" goes. If I'm lucky, I can use the NH400 to make the pieces for the custom spindle block.....
Tom G.