It sounds like you want it to help prevent disease, so you would want one with enough wattage and low enough flow to kill pathogens, not just algae or even just bacteria.
What are the water parameters? I assume it is cycled so I am mainly concerned about nitrate concentration.
What is your water change schedule?
IME new fish may die. I don't think a UV will prevent that, but if you have the money for it go for it, it won't hurt. I have also found that fancier fish are more sensitive, so if you are spending money on the newest varieties of livebearers you may have problems from time to time getting them to settle in.
How fast did the fish die? If it was within a few days with no signs of illness it was probably stress in which case the UV would not have helped.