I think there are a couple things you should consider before you decide on how much cooling you really need.

If you have a 55 gal reservoir, then you obviously have a significant volume to cool. The real question is....how long does the water remain inside the reservoir between cycles? If you're using a flood and drain and you're feeding 48 containers from that 55 gal reservoir on a very frequent basis, then the volume of water will heat quickly when the water cycles into the containers and won't have much time to cool while in the reservoir. The longer the water remains in the containers will greatly influence how much heat is absorbed into the water before returning to the reservoir.

You should insulate your reservoir (Water Heater Blanket) as well as your containers (if possible). If you're using bucket containers, then place plastic over the rocks preventing light and heat exposure to the grow medium (plastic picnic plates work well). Another trick is to put a (small) fan blowing across the top of the water inside the reservoir. This will remove heat through evaporation. You will find yourself topping off the reservoir more frequently, however, the evaporation trick will lower the temps significantly. You should attempt as many of these tricks before you invest in cooling devices. This way you'll save as much as possible by purchasing only what you need.

Lastly, 58F is way too cold. Idealy, you want your reservoir temperature to be at 68F.