Buck regulator frequency
Keep in mind that I'm a neophyte on EMI (and didn't [have to] get any products through certifications), but I've managed to find something in a digikey article narrowly but somewhat vaguely addressing your question:


  Another benefit of switching at a frequency as high as 4 MHz is that it enables designers to avoid critical noise-sensitive frequency bands, such as AM radio. However, there is a trade-off. For example, electromagnetic interference (EMI) can be problematic when operating at high-switching frequencies. EMI from a switching regulator is proportional to the square of the switching frequency – in other words, if the switching frequency doubles, the EMI can increase fourfold. 


Hope this helps.

Also keep in mind that what continent[s] you want this certified on matters. From Maniktala's A-Z book, which I warmly recommend on practical issues like this:


  The FCC spectrum for digital equipment (currently) begins at 450 kHz, while the equivalent CISPR/EN regulations start at 150 kHz. So, FCC compliance can be
  achieved with a relatively small and inexpensive filter. CISPR/EN Class A compliance often requires a filter with at least twice the volume of
  the FCC-level unit. This filter can therefore be up to 50% more expensive. CISPR/EN Class B compliance can require a filter with 3-10 times the volume of the FCC unit, and could cost up to four times more.