I'm curious... why do you want a zip drive built in? I hadn't paid much attention to them in the past, but I thought they were going the way of the 5.25 inch floppy drive.
Since you're looking for a great deal on a low end system, I'd recommend getting the great deal on as close as you can get to your needs. There are a lot of systems out there with 40GB hard drives and 256MB Ram, but the ones that have zip drives also may be harder to get. Just get the computer without the zip drive, and if you can't put one in, have someone you know who's familiar with them install it for you. (Since you're online, I'm going to make the assumption you already have a computer, and perhaps need the zip drive for files you already have.... just take it out of the old computer and put it into the new). Good Luck