I'd start thinking about seat bases, and get a spot picked out for the gas tank and how you want to rig it.
I would want a console along the left side, maybe not full length but pretty long, with a cubby near the back for pliers, tools, and other misc stuff that you need. This is also where your graph, gauges, switch panel, and battery would typically go in a tiller boat. The front half could then be used as storage, a livewell, trolling battery box, etc.
The deck depends on your style of fishing. Vertical fishing or casting is easier done when you're up high, drifting, trolling, or spider rigging can be done easily from the floor.