Nanoe is a type of air purification, it’s claimed by putting a charge on water molecules it does various good things, https://www.aircon.panasonic.eu/GB_en/happening/nanoe-that-improve-air-quality/
The capacity’s given are the volume of water the dehumidifier can remove from the air under a given set of operating conditions. The conditions may vary between manufacturers so are not necessarily directly comparable.
The more expensive units may have a different type of compressor which is quieter and more efficient. You can try to find model information from the retailer.