In regards to your light source, people often prefer an infrared light source so that you do not have a visible light shining down on the track under your light bridge. The cars break an 'invisible beam' of light to register a lap. If that is the look you want, then go with infrared LED's, as long as your sensors under the track are rated to receive infrared signals.
As for connecting to your computer, I can't help on the USB interface cable, as I found an older computer that had a LPT port, and wired it that way. If you have access to an old computer with LPT, it's a good way to go for a computer that can be dedicated to your slot track. You don't 'need' a newer technology computer to run your track timing software.