shopping for a flash

[sprintfan]

I am a beginner with a Pentax ME and ME super. I shoot race cars on the track at a distance of 30-50ft. (mostly at night) with 400 speed film. I have a 28-80 zoom and a 135mm lens. I am looking to purchase a used flash. I have narrowed my search to the Promaster 5600 or 5900 and the Vivitar 285. Am I barking up the wrong tree? Can anyone help me with this? Please reply asap as these flashes are on ebay. Oh by the way...my budget limits me to under $100. Thanks!!! :?: