The P16PRO is basically designed for programming 18 pin PIC's, the P16PRO40 accepts PIC's from 8 to 40 pins. In order to so it has slightly more circuitry, an extra line called Vpp40, which is used to switch the larger chips to programming mode - the only reason for this is to make all chips fit in a single 40 pin ZIF socket.
Given the choice, get the P16PRO40, it's more versatile.
The circuit is available on my website (due to many requests) at http://www.winpicprog.co.uk. I've not seen a PCB layout, but there are plenty of very cheap kits you can buy, which really make making your own pretty pointless.