TBuy the SE370. You can't add needed space later. Also get the Pro package seats. They are worth the additional price for comfort and support. The SE330 is just too tight for most people. I wish I had bought the SE370.
You can put one or two seats "anywhere" inside the kayak. I actually took one seat and turned it around when I go out paddling by myself, to use as additional backrest to my seat, although it's not necessary. You wont be disappointed.
It generally costs as much to ship this product internationally as it does to purchase. Therefore the cost doubles.
I would put a bath rug on the bottom of the kayak; protects the interior, and gives the pup something firmer to grip with their feet.