Wow lots of questions. Many opinions, and these are just mine.
I don't like holes in sumps, but instead prefer submersible pumps. If you do want an in line pump then you can get a great sump tank from Brad at the Aquarium. I have no idea what they cost. Cheapest method is Rubbermaid tubs and containers. If you just want a simple acrylic box, you can buy them, but they are pricey when compared to a glass tank. I would probably recommend getting a glass tank (bigger the better) and would say you should have like a 30 gallon sump? Maybe?
You can keep your micron bag sump, or remove it. Advantages and Disadvantages to either way, more of a preference on what you think will work best for you. I would recommend seeing some tanks, and deciding what you think will work best for you.
Adam