Hi J.,
I have the Nikken Power Pro 5 and love it. It is a 5 stage filtration unit - HEPA filtration, plus negative ions without producing ozone (critical as most others produce ozone which is bad for your lungs). I used to have a Sharper Image Ionic Breeze which I later found out produced ozone. I had another one I bought at Sears and the filter never seemed to get dirty - plus it was noisy. You do get what you pay for with air filtration and since your daughter has allergies, I'd recommend investing in a good one. It is certified by independent labs to remove dust, mold spores, pollen, pet dander, other allergens, organic and inorganic chemical vapors and more.
It has a dust reduction mode on it too which rapidly cleans the air in a room. I can send you a link to watch a clip go over the technologies. There is also a Traveler Unit that is great for a bedroom or in the car (when we go up to the mountains we take it). I still have to dust, but I have noticed a decrease - especially when you vacuum the pre-filter and see all the dust, dog hair and junk it trapped. Better than my body having to filter it all.
One other thing you will want to look at is the products you use to clean your house. This is especially critical with someone with lung or allergy issues. Unfortunately, cleaners are not regulated for the safety of ingredients and you could be absorbing more toxins through your lungs and your skin.
Email me directly for more info (and a clip you can watch). Happy to show you the cheapest way to get it as well.
Take care,
S.