I have 2 surplus Sharp air purifier + humidifiers units. Each unit can function as a air purifier alone or with additional humidification (great for dry Beijing winters). They have been used for five years without any problems and all filters have been replaced in 2014 (manufacturer guidance is that the main HEPA filter lifetime is up to 5 years).

According to product manual, main features are:
- traps dust with washable pre-filter
- decreases odors
- reduces pollen and mold
- humidifies (optional)
- for rooms up to 254 sqft / 28 sqm

The two units are for sale together for 700 RMB.