Unlike most brands Onkyo supplies a real receiver with their HTIB packages which has the advantage of giving them an upgrade path in the future. That said the weakness of every HTIB system that I've run across are the speakers and subwoofer and I'd have them listen first and see if they like the sound. It would be nice if they could piece together a system instead but prices are so high up there that I'm not sure how doable that is. One approach is to start with a nice 2.0 or 2.1 system and add to it over time as money allows.
Keep in mind that the room's size (6300cuft) and square shape will be a challenge for any system. The subwoofer needs to move enough air to slightly alter the air pressure in the room to do it's thing and in the US you'd be looking at a $1300-1500 subwoofer for a room that size plus room treatments to deal with issues caused by shape of the room. I'm not an expert on room treatments so I'll leave that to those that are.