Rye,

Unless you get a used G540 in the UK then it will be cheaper to get individual drives and a breakout board. I like my g540 and have used it on my 3040 cnc ever since I found that the original drive sucked.

What you refer to as an external parallel port is in fact called a motion controller. These do a bit more than just replicate the parallel port. Whichever brand you do go for, we generally agree that the ethernet type is more reliable.

As for the motors, don't bother, with the right drives they will do just fine.