If you use old wiring, that will be as good as the system sounds. If you use poor quality mics, or amps or speakers, it won't sound any better than that component. Manufacturers such as EAW, Meyer, and JBL design boxes for specific applications and then supply tech notes to installers indicating do's and dont's for that cabinet's use. A quality system will be more expensive up front, but over the course of a few years becomes the cheaper way to go. They last longer.
Maintenance costs are lower.
You don't have to pay for someone to come out over and over again to make changes due to poor coverage. And most importantly, the quality of sound will be many times over worth the price. Example Lake Community Church, Alexandria, Minn. did an excellent job with a tight budget.