Since you are a Linux user, you might want to know that Linux Weekly News just the past few days has run a couple of features on open source CNC.
(The second article, covering FreeCAD, is currently available only to subscribers until the middle of next week.)
The first LWN article I linked has links to several programs for toolpathing.
I use OpenSCAD extensively for designing parts to 3D print (additive manufacturing) but not for milling (subtractive manufacturing). You can use it, though. FreeCAD has some fairly extensive CAM capabilities, but it’s not the tool I’d use first to generate toolpaths from STLs. It has a fairly steep leaning curve. That said, it has been my primary tool for generating toolpaths. But you could try some of the other programs he links to, like the browser-based Camlab.
For whether that particular stepper motor will work, probably… Certainly within the limits of the Smoothieboard. Honestly, the steppers are the last step of the build. The fixtures to mate the stepper is the same for any stepper of the same size class. I would figure out everything else before buying any steppers!
I seem to have forgotten to point out that there are two kinds of torque: a single torque rating (140.2 oz/in for the stepper you linked to) is holding torque: it’s how much force it can hold still at when it’s being held to a full step. The torque it can apply while moving is lower, and the higher the RPM the lower the torque it can apply; see the torque curve at the bottom of the page you found. The best you get is about 100 oz/in at around 40RPM. That will be with full stepping, and you will want to use microstepping for smooth movement. Full stepping, 1/2, and even 1/4 microstepping sound horrendous and make the machine vibrate. 1/16 microstepping is what you really want to use in most cases.
You might need a larger stepper for Z than for X and Y. Usually you can put up with Z moving more slowly than X and Y, though; moving Z at lower RPM will be OK.
For those steppers, you would need 1/4" to something (6mm? 8mm? 10mm) couplings between the shaft and the screw. Diaphragm couplers are typically the best; lovejoy/spider couplings second.
You’ll have to design and make fittings that go between the stepper and the mill. This should have a central hole to indicate against the round section around the shaft. This is what keeps the stepper perfectly centered. Note the tolerances for the motors themselves: for NEMA23, it’s 1.5" ± 0.001"! Then you’ll need four mounting holes. On NEMA17 motors, the mounting holes are usually threaded M3 and the screws go through the part to which you are attaching the motor. On NEMA23, the holes are typically 5.1mm holes intended for M5 screws to screw into the part you are mounting to, with the screw typically a hex socket head screwed into the part to which you are attaching. (This is typically more convenient for a CNC machine.)
Or, a better idea, you buy something someone else made. For example:
I wrote lots of advice on all the things you would need to do, and then discovered this kit:
That uses an 8-bit arduino-based Synthetos gshield (see #controllers:tinyg) controller instead of the Smoothieboard, but it shows what components are there. The Smoothieboard is a good idea; 32-bit processors are better for cutting arcs. The kit is basically adapters like the amazon link, three stepper motors, electronics, and a housing.
Here’s another writeup:
The instructables author purchased from mbbilici. I don’t see a MF70 kit on his site, but I have purchased from him before and recommend his work, and he has responded to email. You could ask.