I wouldn't buy motors from a website that says they'll send you one or the other, whatever they have in stock.
The best performing drive for hobbyests is the Gecko G540. Check out the kit with the 381oz-in steppers from Keling.
As for screws, go with either 1/2-8 2 start acme (150-200+ipm) or 1/2-10 5 start acme (250-300+ipm). Available at www.mcmaster.com or www.mscdirect.com
Get antibacklash nuts from www.dumpstercnc.com
You can download Mach3 and see if it'll work on your laptop, but it doesn't work on most. It's free to download and use for up to 500 lines of code, so definitely download it and learn how to use it now.
Actually, Mach3 will probably run OK, but it may not run you machine OK. I tried using my laptop, and the motors run extremely rough and stop and start. With my $20 1Ghz Dell desktop, they run great.
It should. :-) But you'll need a CAD or design program and a CAM program to create g-code for Mach3 to run.Last question for this post:
I built the Machine, order the electronics, install and wire electronics, load software, connect electronics to computer, run program, CNC will function?