well look on eaby . I reckon a pentium 3 266-500mhz with 128 mb ram hard drive of around 4gb. Also a cd drive is not essential as you can put the photos on usb memory sticks and use theese for that function that the cd would use . also that would be cheaper as there would be no need for a new cd every weekend
Another possibility is to buy portable dvd players and produce the slideshow as if i they were a dvd movie then use the portable dvd players to show off your work .This may be cheaper as well
EDIT>
Here found the kind of thing that would do the job
http://cgi.ebay.com/IBM-ThinkPad-262...QQcmdZViewItem
Cheap usb flash disk . 8.99 for 1 gb
http://www.newegg.com/Product/Produc...82E16820191053