Filter before or after switching regulator?
It really depends on what you want to protect:  The input power from the buck converter, or whatever is being powered by the buck converter.  

A buck converter primarily causes ripple and noise on its input, due to the fast rise times of most switches used in such converters.  You generally place as much quality, ceramic capacitance as you are willing to pay for or have room for on the buck's input, and you place that capacitance as close to the switching current loop formed across the high and low side switches as physically possible.  If you aren't extremely careful with the input filter, you can cause all sorts of instabilities in the buck converter, or make the input ripple even worse.  300Hz is nothing, the converter will filter that out like it was never even there.  

If your input power is already noisy, and you don't really care if it becomes even noisier, then let the buck converter deal with the noise first.  It is going to inject wide bandwidth harmonic noise starting at the switching frequency and the harmonics thereof up to as much as several hundred MHz (depending on the rise times).  Your switch node will ring well into the MHz range due to parasitics.  

If you put the filter after the output, then you will attenuate all of this as well as whatever noise remains from the input.  Unless it is noise on the same order as the converter switching frequency though, I don't think very much of it will make it into the output.  Some always does, but it will be drastically attenuated.  By how much depends on the specific regulator and its ripple rejection across the frequency range in question.  

Long story short, if you put a filter on the input, this will filter the input from the buck converter more than it will filter anything on the output, while putting it on the output will almost entirely filter the output.  It is not uncommon to have input and output filters, but if you don't care about even more noise getting dumped into your input from the buck regulator, just filter the output.  

Also, slap a nice series connected ferrite bead on the input and output.  They're cheap, simple, and can work wonders on those higher order harmonics.  No one wants those.