Talon Leighton

I'd definitely avoid sitting it directly on the plywood. If it were mine, I'd put down a proper hearth first, then use a non-combustible base if the insert needs extra height. It's much safer than relying on tile alone.

For the chimney, get a flashing kit that's designed for metal roofs, along with a storm collar and high-temp sealant. It makes sealing around the pipe much easier and helps prevent leaks.

I like the idea of mixing those different stove designs, but I'd keep it simple to begin with. Get the basic stove working properly before adding the oven and extra cooking plates. Reclaimed bricks are a great choice for the main structure, but I'd use heat-resistant materials anywhere the fire is at its hottest. I'd also make sure there's an easy way to remove ash and clean the flue—it'll save you a lot of hassle later on.