If you do not need to have a plotter you might also consider a large format inkjet printer, like HP DesignJet. This would be more versatile in use than a plotter, and easier to maintain. Several old machines have a serial port or Ethernet option and a Macintosh driver available. Some are PostScript ready, what enhances the output quality of vector graphics, usually. If you find a cheap DesignJet without a PostScript option, you might use a second Mac running a raster image processiong software as a print server (a LC 475 or better does a good job here, after configuration even without KVM and even without VRAM). This is only suitable if your CAD software supports output to a standard printer, of course.