Advice on my new metal shed please

Yeah it should be fine but 18mm ply would obviously be better but more cost effective!

Depends on how your concrete base is made (above or below ground level). It should be higher. Then cover with bitumen (cheaper than other analogues) to prevent the ingress of moisture into the inside. Install and secure your metal shed. Then you can put a damp proof membrane so that all edges of the membrane looks up when you installing floor beams support. The plywood of 12 mm is not strong enough. More better to use 18-22 mm but it all depends on the distance of the beams at the centres and what you intend to store in the shed. Before laying plywood needs to be painted as this is the sheet material that absorbs moisture from the environment and expands. Other sheet materials such as OSB or P5 green tongue and grooved moisture resistant chipboard are not as durable as plywood. Also very important moment- ventilation of your shed.

Hi yes 18mm would be better but I would also try and raise the ply off the floor thus leaving enough space so air can circulate under the floor

Metal sheds are bad for condensation they sweat and drip,if price is not and issue i would use 18mm marine ply and then fibre glass it with a good coat of topcoat resin paint, 25mm insulation inside the roof will help stop it condensating and dripping.