DIY. it's fully customizable.
Oceanic 30G cube with a 10G sump? i know it's not an all in 1 package but if i learned 1 thing from my 12G nano cube was that i could never control the flow without having to compensate something, whether it was an ugly outlet sticking out on one end or having to add a microjet to help keep detrius suspended...
either way, it was a constant struggle to keep the tank clean. weekly 3 gallon water changes in a small tank without a sump is also very difficult.
IMO, it would be best to design the entire system from scratch. you could (with something like a 30G cube) design an overflow outside the aquarium (a glass overflow "box" per say" and give yourself more room for aquascaping.
also, if this is your first marine tank, i would advise highly against a nano, as the small water volumes makes the tank and its inhabitants more susceptible to swings in both physical and chemical parameters of the tank. Nanos are far from "self sufficiency". if anything, i see the new nano trend to be nothing more than a "quick fix" solution for novice/beginner aquarists and are often sold as a "set it and forget it" solution to maintaining these sensitive organisms.