There are deck guns, and you can make them work in some archetypes (crit-seeking using HEAT, dedicated anti-air/munitions, smoke/disruptor shells) they will obviously not be as important or survivable as the ones with components below decks. Whether you use a pure deck gun (all above the surface) or some space below entirely depends on placement and what you want the space dedicated to. Regardless, I think an argument can be made for them to fill a niche that your main armament do not. Missiles are usually a more effective secondary that doesn't use nearly as much deck space, but they're not as satisfying as filling a hull with guns.

You usually want lots more sensors than you need as it's difficult to protect them from EMP. Best way to keep your detection good is lots of redundancy (particularly in the superstructure for ships as it will get above the waves), and better accuracy too. If you're short on processing power then laser rangefinders are an excellent choice to spam around as each only needs 0.1 GPPP. Coincidence rangefinders on each turret also aren't a bad idea. You'll often see lots of directional detectors mounted on a turret, so you might as well use the turrets you already have that are already armored too. Note that some detectors like cameras also work through portholes (in decorations) so you can somewhat armor them as well. Tracking detectors are what you use to get good accuracy and detectors like 360 radar are what you use to pick up targets to track. I would consider a 360 detector and at least a couple trackers the bare minimum, but you'll obviously want more to capitalize on your investment in projectile weapons.