In all honesty, you need to consider what controller you'd like to use.
The parallel port is largely dead (for Mach, anything newer than Win7, or any 64bit OS, it just won't work), so I'd be considering an external motion controller.

If all you want to do is run 3 axes and a spindle, then there are lots of options.
If you want to run 4 axes, and other functionality like a tool changer, then it does reduce options.

In terms of Mach, Mach 3 is probably still the most widely. It is no longer developed, but there is still a quite large community supporting it, and I doubt it's going to disappear any time soon.
Mach 4, I do think has improved greatly, but there doesn't seem to be that much community support for it.

If all you want is a basic 3 axes mill, options to consider are Centroid Acorn (beware of the limitations of the free software), CNCDrive (either UC or AXBB interfaces) and their own UCCNC software, PlanetCNC, or LinuxCNC.
There are also the plethora of other external controllers that rely on Mach3/4, like Pokeys, PMDX, and the cheaper CS-Labs controller.

If you want to run a more complicated machine, then you need to look at more expensive/complicated options, like CS-Labs (they do have their own software, but I've not heard a good review of it yet...), Dynomotion KFlop, LinuxCNC with suitable Mesa Card(s), and lots of other options which I can't think of just now.