Great score!!!! That's a heck of a good price.
THe piece your not sure of is the bio tower. That is called DLS (dual layer spiral) inside of it. It was used in place of bio balls for surface area for bacteria. It works very well but also traps dirt and eventually becomes a perminate nitrate and phosphate problem. In the pic of your overflow box it looks like DLS was being used for a pre-filter material which was common several years ago as well. For fish tanks this type of set up works well but I'd replace the DLS with bio balls. If your going to do more of a reef type aquarium, this might not be the best way to go.
Does the bio tower just rest inside the sump? If so you could not use the bio tower and modify the sump to become a "marsh" or "fuge" and use the over flow box to deliver water fom the tank to the sump. If you want some help or ideas, stop by next time your in the neighborhood. We'll come up with a plan that will work.
See ya...Joel