The parts you need for a cruise retrofit on a 2003 or 2004 Boxster or 996 are:

1 x 996.613.219.10.EWC - 4-Lever Stalk Unit
1 x 999.652.972.40 - Connector (for Cruise loom to stalk)
1 x 999.652.973.40 - Connector cap (optional - it helps to polarise the connector, but isn't essential)
4 x VW 000.979.009 - Wires (VW call them Repair Wires)
1 x Scotch-Lok Connector (to connect to wire to fuse B7) - or solder & insulate
1 x length of wire about 1 metre long to extend the VW wire to reach the fuse box. (I use a wire size of 16/0.2 which looks about the same size as the VW wire.)
Sundries such as insulation tape and/or loom tape, some small cable ties, etc.

If the stalk unit you get is second-hand then it probably has the lever caps (with the symbols and writing) on all 4 levers. If it is a new one, they don't always supply the cruise cap. If you need it, the part number is 996.613.505.00.A01 for gloss black, and 996.613.505.10.A05 for matt/satin black. You can use the other caps from your existing stalks.

If you are adding OBC you will also need:

1 x 999.650.513.40 - Connector (for OBC loom to stalk)
4 x VW 000.979.009 - Repair Wires (in addition to those above)

http://www.pelicanparts.com/techarticles/Boxster_Tech/92-ELEC-Cruise_Control/92-ELEC-Cruise_Control.htm