The curved ribs are just to give a bit of stability. With straight ribs, when you load thicker / heavier stock, the ribs flex while the substrate is loaded.
Also, with straight ribs there is the small chance that a straight cut in the Y direction may follow the same path as the rib underneath. With a curved rib it is not possible.
The 1500 x 3000mm cnc plasma table at work has got straight ribs, but I see the 1200x2500mm version of the same manufacturer advertised in Popular Mechanics has the curved ribs.
So, either will work - they just have to be saw-toothed on the top. We use 3mm MS plate for the ribs, and they last a long time - even with the plasma having a rather busy schedule.
Depth is not that big of a deal. About 50mm deep should give you enough space to fit suitably strong ribs.
You will have to use additives in the water even with a stainless system - in order to prevent the build-up of algae. (Or you will have to drain and re-fill regularly)
The stainless ribs will also be expensive, considering they are a consumable as well.
We use water solluble oil in the water, and drain, filter, and re-fill the bed once per month. Top ups are supposed to be with the oil/water mix, although human nature means it is usually just water. The monthly recharge sees some fresh oil added.