You mentioned that you had a power amp available for the main speakers. That would be fed from the MAIN output of the mix desk via either the XLR or the 1/4 phone aux sends. Mow many separate monitor channels do you have available from the desk? Each input section will have a man fader (slider), a gain control, various EQ controls and monitor sends. Some desks have one monitor send and other can have up to eight. You would take a monitor send from ONLY the preacher's mic channel into either a self powered speaker in the remote location using shielded cable or feed a small power amplifier and run a speaker level signal to a PASSIVE remote speaker.
As far as directly connecting a video recorder to the audio channel to record the preacher, it can be done BUT you must use a level matching pad between the desk aux output and the video camera input, UNLESS the audio input on the camera is clearly marked as a LINE LEVEL input. Otherwise you will be severely overdriving the input to the video camera. Pads can be bought at Rad Shack and are red plastic barrels with various I/O connections. There is a level dropping network inside the barrel.