Since their purposes are essentially different,the necessity for either one or both is completely up to you. The diffuser is used to scent the air and/or deliver aromatherapy. The air purifier is to clean the air of harmful particulates like smoke, dust, or noxious odors.
If you put what you just said in the ad, I see no reason why you would need more than one. Be prepared to pay more for the initial cleaning. The same person or team would have a much better idea what would be needed after the initial cleaning than... more
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