The depth of the step would need four boards at 1500mm. You'd need just over one length of the Ekodeck to cover the top. So if you bought two lengths that would cover the top and front face. You then need to construct the internal frame of the step with Treated Pine Outdoor Timber Framing 90 x 45mm and you'll likely need several meters worth.
Depending on how you go about constructing the frame I would estimate you would be spending around $200 on materials.
Please let me know if you need further assistance or had any questions.