Have a look at
www.blackbox.com.au they have options for serial and usb, but I could not see one for parallel port. I'm excluding the print servers as I don't think they will do what you want.
I think the right laptop/PC with everything you want would be better otherwise you are probably looking at multiple boxes each requiring power and a network connection to a small switch. Read lots of cables. And it would probably cost as much or more than a small laptop.