common mode chokes and their selection
C1 is your first line of defense against differential noise coupled onto your wires, and your last line of defense against your device sending signals out into the world on those wires.  Twisting the two wires tightly will help with differential noise as well.

If your buck supplies 4 A, it does not necessarily draw 4 A (as a linear regulator would!).  If you have a specification on its efficiency, and you have good knowledge of its operating point, you might get away with less.  But 4 A should certainly be enough, so that is a good 1st point.  If you can't get one you like rated at 4 A, then sharpen your pencil (so to speak) and figure out exactly how much current will be needed.

Regarding the frequency for the common mode choke, the best option is to build it without one and measure the noise.  Then select one that is best suited for where noise is the worst.  If that is not feasible, then just try to get it below your circuit operating frequencies.

Do you have a separate chassis / earth ground, or is your negative coming in grounded?