I would get the Aquaclear 70 (they run about $45). I have two on my 55 gallon goldfish tank. I had the Penguin 350 and felt like it didn't have as much biological capacity as my Aquaclear. They are actually constructed more like a canister filter than any other HOB and offer the best options for bio-filtration. Forgo the carbon and get two sponges of different pore sizes (cut the one the filter comes with in half for more room) and get a second bag of bio-media. Ideally a canister is best, but they are also expensive. In a goldfish tank you want to maximize the surface area for the BB to grow. This is especially true for small tanks.
For weekly water changes, you are looking at either a single 50% or two 25% throughout the week. Neither of these will hurt the cycle. Only a 100% will. In fact, I do weekly 50% water changes on all of my cycled tanks.