Peter it depends on the type of BOB that you use. Some use very little voltage or current at all. The problem a lot of those have is making sure a good signal gets handed on to the Drivers. With you wanting to do steel, I would seriously look at a slightly higher end BOB which in turn will most likely require a higher voltage to run. I have gone up through the BOB line starting with one that I won't mention and then moving on to Roy of DIYCNC PPCPSv2 unit (gave that away to a useful home) and then on to his system 4 board set up (Which I have for sale with extension cards but not sure it is the best for your needs even though it is opto-isolated). I used that with a borrowed USB Smooth Stepper and got huge improvements in control and accuracy (Parallel port on that computer has issues).

Currently (and pretty much the last one I will ever need) using a PMDX-126 board (mains powered) with an Ethernet Smooth Stepper (was in the States and could buy them without the huge postage costs). The difference is massive. Now I am also running AM882 drivers which also make a huge difference. You are looking at running servos and therefore really want to keep as clean and solid signal reaching them as possible. So look around and ask a few folks on what would be best for you. Good luck with the build.

Michael