If you want to clean everything off, fill it with water, dump a gallon of vinegar in it, let it sit for a few hours with circulation, and you'll be able to wipe it right off. It won't hurt the tank or silicone. Go with as large of a sump as possible, it only makes things easier to setup and maintain. For instance, I have a 180g tank, and a 125g sump. Now I can go that large as I have everything plumbed to the basement, all support equipment sits down there, but the extra water volume is a good thing.