Definitely make it fit through a door.

I would suggest that 12mm ply is the absolute minimum thickness and sufficient only if you have a big structure of ribs underneath. Once filled with water, the sink may need to support 100kg+ and you want to make sure the structure is good enough for that without the bottom falling out or the legs folding up under it. Most of the sink designs I've seen are 19mm marine ply.

On marine ply, there are two aspects: the glue used and (more importantly) the lack of internal voids. Non-marine ply can have plies overlapping at joins, which leads to little voids, in which water can sit and fester. Yes you still need to seal it, but it lasts better than normal ply in a damp environment.

You can buy the epoxy used for fibreglassing, mix it up and slap it on with no glass mat, and there you have a waterproof coat. I believe there are less-nasty (and cheaper) versions based on polyester that are maybe easier to work with.

I had big plans for a sink but ditched them: I just use a classic small stainless sink in one end of the darkroom/laundry bench, with the Jobo sitting just next to it. No trays 'cos the Jobo will process prints to 16x20". Much prettier than an epoxied plywood monstrosity and (I would assert) much higher WAF even if my Jobo is always parked in the laundry. But then, I do 90% of the laundry work anyway so she doesn't need to care that it's also fitted out as a darkroom.