i would try to think about your demographic some. Is it a lot of college/younger people? Made up of more middle aged, elderly?
I think a single ds3 will be perfectly fine because ive heard of some cable companys running entire smaller citys off of a ds3, unless the majority or high usage users then it may be a closer call. If your budget can handle it then as suggested start out with 2 partial ds3's and you can always expand them if needed.
