I am not 100% sure as I am fairly new at salt two, but this will at least bump your post if my thoughts need to be corrected.
1- You still need to have a form of biological filtration, however whether or not that filtration is in your tank or sump. If it were me, I would get the same amount of live rock as I would without a sump, because I like the look of it in my tank, especially once it starts to grow stuff.
2- as for it being necessary, first step would be to check your tap water. Although you cannot check for everything, your water treatment plant around you is required to perform checks on many many chemicals. This will give you an idea of what is in the water and you can work from there. That being said, if it were me and my tap water was ok I think I would be comfortable using tap water for everything except a reef tank, because they are so much more sensitive to water. Also depending on the size of your water changes and the size of your tank .50$ a gallon will add up fast, and will quickly offset the price of at least an RO
Just my two sense, and I am sure you will here from a bunch more people soon.