panels

Hi, Looking to fit upvc ceiling panels in my shed. I've seen 5000mm amd 3000mm lengths and either 250 or 300mm wide. I would probably go with a 3 or 5mm thickness. The 3000mm would be best as it will fit through the door plus I can join it in the middle. Is there a product for joining...

I haven't used this panelling before, haven't done much tiing either. Shower panels look like the way forward if I can get some for above the bath. Does anyone supply custom panels? (probably not laminate). Thick enough to be rigid. For walls above long side and each end. Approx.160cm tall...