RFI, Radio Frequency Interference, and EMI, Electro Magnetic Interference, are interchangeable terms depending on the source of the emissions and their interaction with other electronics. For most intents the two terms are one in the same *and yes I expect others to point out the finer aspects* in layman terms.
Typically interference caused by other sources will occur when the types of electronics involved use frequencies that are very close to each others operational frequencies, or harmonics, and or the power output of the offending emissions is high enough to induce reactions in the electronics of the device being used.
For most metal detectors, VLF devices, other radio sources like FM broadcasters don't cause any interference with detectors because the fundamental frequencies both use are very far apart in the radio spectrum. Detectors use VLF frequencies ranging from 5 to 50 kilohertz where as FM devices can use VHF 99 megahertz to 121 megahertz or HF 2.4 gigahertz.
About the only time rebroadcasters are a problem is when the output power is high enough to cause problems or the transmitter is placed to close to the detectors electronics. But its not very common.
Long story short you'll probably have no problems with wireless. I would suggest you use a device that operates on 2.4Ghz as they cause the least interference in my experience but many people have used VHF devices successfully. 2.4Ghz devices often offer frequency hopping which reduces problems with other transmissions on received audio. VHF devices have to be tuned to open frequencies to reduce bleed over from common radio sources like radio stations.
