What type of setup will this be intended for, if I may ask?
Most of the time, UV is not needed for regular reef system, but if you can convince yourself that you do need one, it should be at least 80W bulb to handle that much flow.
Split your flow line with a T and tap the exit to the UV so you can adjust the flow. the slower flow the better. And id also get one with a cleaner built in, taking those things apart can be a real pain, and you can guess that with the flow being reduced you get quite allot of precipitated depositation on the quartz tube. My recomendation, fish only.