If you already have a Mach 3 license, then I'd suggest a UC100 as the easiest option given you already have a parallel port connection.
Off course, there is no real reason not to use an old PC and parallel port if you want to keep cost to an absolute minimum. Plus you could even use LinuxCNC with that same setup.

It all depends on how much money you'd like spend, how much you want to modify things, and how much time you want to spend getting the setup working (I'm generally wary of things like that beaglebone setup, as I've often found key details are missing, so unless you know the system well, it can take a good bit of reading up/experimentation to get things working).