U.V sterilizers do not negativly affect your water quality, in fact, they can actually raise your ORP level.
You don't need a U.V. sterilizer. The advantage to using one is that it it may decrease parasite out brakes. That's really about thier main advantage. If you keep fish that are parasite prone or have a situation that your aquarium is not as stable as desired, it may be a consideration to have one. Likewise, if you keep fish that are sensitive to medications it may be a good idea. I use them and like them but by no means are they absolutly necessary.
It's an issue of good better and best. A well designed system might be a little better with U.V. sterilization. On that size aquarium, I personally would use a 30 watt.
Joel