I don't know oe a specific set of plans being posted online here. The things you need to concider is: How much stroke do you need? If using a THC like Candcnc's MP3000 it needs to be able to travel somewhere in the range of 150 IPM.
for 6" travel or so your lead screw needs only a bearing (best to use a double row angular contact bearing) on 1 end usually the top to limit endplay of the screw. Screw/bearing assembly will beanchored to the back or base plate along with the motor. Flexable coupling between motor & lead screw is common. You can find them from almost any CNC parts vendor. Check out the various parts vendors advertising here on the zone. (TEA come to mind right off but there are many others as well as EBAY)
Ball nuts usually have a threaded or flanged end to mount them to your moving part of the Z. A couple small pieces of linear rail & a couple bearings makes up th sliding surface between the base plate & the movable plate. of course the center of the ball nut & center of the lead screw must be centered between the 2 plates.
The floating part for the torch mount only needs about 1" of movement & a secure, safe plase to mount & operate a limit switch.
Here is a very good example in pictures. Look specifically at posts 1, 16, 18, & 29 : These picture shuold pretty much explain everything.: My 4X4 table build
Go through the build logs of plasma machines & router machines as well. You should find something to meet your needs. There is everything from entirely home made using pipe & skate bearings to any kind of linear rail out there.