You use a regular 2m receiver attached to the doppler - it can be the
receiver in your mobile rig, HT, scanner, etc. The selectivity is the same
as for normal ham 2m use. The doppler will indicate a direction based
on the signal it is receiving, whether it is the one you are trying to find
or not. So you have to be listening to the audio to know what it is
hearing. (I think some dopplers include an audio output with the doppler
tone notched out of it.)