Parts for HPS system?

Do i need anything else for an HPS lighting system other than the ballast and bulb and power cord?

do you mean, should you have other parts to repair your system in the furure i.e an extra cap and starter or do you mean like the fact that you didn't mention a reflector or a socket..etc...i can list every part you need to build an hps system if you want but i'm not sure of your question?

if i buy an HPS ballast and a bulb is that enough for the light to come on?

if the ballast has a power cable and a cable going to a socket that you can screw the light into then yes when you plug the ballast in the light will most likley turn on granted it's a good bulb and ballast and that they are compatable...

Well you need a hood or tube that will house the bulb.