There is nothing wrong with packed tight so long as you allow for air flow. Just do a trial layout, like you are doing in the picture, with the equipment that goes in the box. You may also add a little extra room for the unexpected. Don't forget to design in the relays, terminal blocks, breakers, disconnects, fans and wire ducts.
The price of the cabinet will be proportional to it's size. I like the Yuco IP66 enclosures. A 16x12x8 will probably be the one I use on my next project and it is $118 on Amazon.
Remember, vendors spec the outer dimensions of the box, not the panel inside.