A uc100 has advantages., As do uc300usb / eth & 400eth
Note the uc300eth does not at this time have a mach3 driver, but it should do as all the others do.
Even if you are a mach3 stalwart, best to consider mach4 and others when laying out a load of cash
http://cncdrive.com/products.html, with webshop. There are local dealers across the world.
Even if you are a mach3 stalwart it is worthwhile at least considering everything at the time of expending money as mach3 is stagnant whilst mach4 (and others) may surpass mach3 in time.
Motion controllers can be a bit of a minefield, as some don't do everything that the parallel port does under mach3, and with mach4s development being slow and steady there can be issues with plugins when a new release lands.
Your cheapest option may be a second old pc with 32 bit windows 7 (or xp...as I run)... but don't loose sight that mach3 is stagnant and at some point a lack of old hardware and software may render it obsolete (but not for a while as w7 PCs can still be found)..... try to find a i386 with w nt + drivers if your software is only compatible with that version gives you an idea of the future at some point