What are you planning to use this machine for? (Let's talk about the first one; you'll know a lot more by the time you get to building the second.) If it's intended for cutting wood, then speed is something you want. Wood needs to be cut relatively fast, or you start getting burn marks - it can even catch fire if you go extremely slowly. Speed in stepper motors is not really a function of the holding torque (300 oz-in in this case), although that's the number people tend to focus on. It's more closely related to inductance - lower inductance at a given voltage makes for faster motion, and high-inductance motors can perform very poorly if paired with a low-voltage power supply and drivers. A random 300 oz-in motor might be fine for your machine - or it might not work at all.
It's usually a good idea to build your frame first, before purchasing motors and drivers; things can change in the middle of a build, and you don't want to be stuck with stuff that won't work. Also, it will give you more time to research this - many people buy motor/driver/power supply "kits" on Ebay consisting of wildly mismatched parts and then complain when their machines don't work, or work poorly.
You can probably find an older computer with a parallel port for free, or close to that. These are perfectly adequate for running CNC machines, and save your good computer from being tied up on long part runs, and you won't have to strip it of all the extraneous functions that interfere with running G-code. If you want, you can buy adapters to run a router off your USB port, but that will likely cost more than the old PC, and you'll still need a computer to dedicate to running your router.