You're not going to get a pc with 2 parallel ports off the shelf, and you probably wouldn't want to.
You can get parallel port adapters that will install into a PC's ISA or PCI slot. If you use one of these then if there's a crossed wire somewhere the card gets zapped but the PC motherboard doesn't. maybe a consideration in a more serious CNC use. You can get cards with multiple parallel ports too and two ports is common. For example do a google for 'PCI parallel port card' and you should find a bunch of single and double port cards. Prices start around $20 - $40..
Selecting the card depends upon whether your pc has ISA slots or not. Older pc's do, newer generally don't. ISA cards are sometimes cheaper, PCI card are more expensive and faster but that wouldn't affect a plasma machine. PCI cards may plug and play in windows easier if thats a consideration.
Other than that, what PC you get depends upon what Software you're running. If it's DOS based then just about any old thing would do, but you'd have to manually configure the cards. If it's windows then just get what the CNC software vendor recommends in speed and add the Parallel port card. If it's a dirty environment a surplus industrial PC might be worth a look if you got cash...