I will use a retail Arduino with a DMX master circuit run into the computer with usb.
I'm not really sure if you can do that... Of course you can upload your DMX sketch from USB and run a pre-programmed sequence, but I'm not sure if you can "run the show" from the computer that way. You might need a "retail" USB-DMX controller...
Can I make the slaves with a hackduino or do they need the serial chip that the retail versions have? Wouldn't it then be more cost effective to buy retail arduino's for the slave units and attach the DMX circuit to them if the hackduino wouldn't work?
I don't know anything about the hackduino, but for DMX you need a special RS-485 chip/circuit in either case.
If you don't already have some DMX equipment, I suggest you get a DMX light (or something) so that when you have trouble, you can use the good equipment to determine if the trouble is on the master/controller side or the device/slave side. Or, you might end-up building something that you think is DMX, that doesn't work with any other DMX hardware. 