A sump is always a good idea. More water means more room for mistakes, and there are always mistakes. I started my 36 bowfront without one and soon wanted one as I wanted corals in my tank. It gave me room to house my protein skimmer. Now it has a refugium, pod population, and my heater in it. It sounds like your LFS
guy knows his stuff. As for the deep sand bed, i'll be honest those frighten me. The chance of accidentally releasing toxic gases trapped under the sand? That is one I'll avoid.