One of the things I liked about my Fuji 4900, which was an ancestor of the S602, is that it would work with any flash, including my studio lights.
You can put any flash on that unit. If you get a flash with Thyristor circuitry, standard on most flashes since the '70s, the flash will take care of the proper exposure.
What you do is pick a power level on the flash. There will be a scale on the flash that says at this power level, use this f-stop and this range of distances. So you might get f 5.6 from 6 to 20 feet.
Then, set your camera to f 5.6 and stay between 6 and 20 feet. You'll get the correct exposure.
If you have a dedicated flash unit for an SLR, any brand, that'll work. I used a Nikon SB28 and it did a good job.
If you want a specific suggestion: Vivitar 285 or 283. Get a new one, older models had very high sync voltages which could damage your camera.
Hope that helps.