How much will a switching regulator influence my RF transmission?
Assuming you do a halfway decent PCB layout, following the layout recommendations in the DC-DC converter datasheet, and use a ground plane to avoid nasty loops, you should be fine.  Make sure you follow all the decoupling recommendations for the radio module.  

Also pay attention to the switching frequency of your DC-DC converter (fsw) as compared with the frequency f0 and bandwidth BW of your RF signal.  You will most likely see noise spikes at f0 +/- fsw.  If the BW of you signal is wide enough that those spike occur within your signal, you may have increased risk of interference.  

Unfortunately, there's just no way to calculate these things.  My experience is that you will probably be ok, but there's no way to say for sure other than trying it out.



Also - From the datasheet linked, section 3.1:

"...to ensure a correct operation of the built-in voltage regulators"

As implied by the above quote, it's possible that the radio module already uses linear regulators on the most sensitive parts of its circuitry, which would help you out a lot.