My biggest tank is only a 55, but it has a 7 inch plec and three 4" spotted raphael catfish, along with some smaller fish. My got a AC110 hob filter, and I think it does a great job. (rated for up to 110 gallons, I always get a size bigger when buying filters)
Those AC 110 filters can be purchased for around $40. I think that a 125 gallon tank would be just fine with 2 of them. However, if I had such a large tank, I might want it against the wall, with all the wires and stuff hidden in the cabinet instead of crammed behind it. I mean, if I was gonna spend the $$$$ on such a "showpiece" tank, I would want it to look an nice as possible.