Hi Andy,

Ok what you have there are 3 motor drives and breakout board that takes pulse signals from a 25Pin parallel port connector.
To use USB connection then you need some other way to generate the Pulses that the parallel port would normally provide. Unfortunatly due to the nature of high number and critical timings of these pulses you can't just use any old USB to Parallel converter as USB on it's own is far to slow.
Instead you need an external device which takes signals from the USB and buffers them along with other trickery and then generates timing critical pulses reliably and that's why you need the UC100 or some other External motion controller. These external motion controllers still connects to the 25pin connector on the Breakout board which is really just a distribution device to make wirirng easier but provide much better quality pulses at a lot faster rate than the parallel port can provide so this increases reliabilty and performance.
Unfortuatlly because of the complexity and critial nature of pulse timing there is no cheap way to get around the parallel port.

Regards what you have hardware wise then from the few pics you provided you should be set to go other than a little rewiring for maybe a E-stop etc.

Hope this helps.