Recommend an Air Purifier?

I need an air purifier but I'm not sure where to start. I have allergies to dust and mold.

Stuff seems to grow mold here faster than it did at my previous apartment. For instance, at the old place I could sprout some seeds on a damp paper towel inside an almost-closed baggie and it would be fine. Here, it starts growing mold within a day or two. I've harassed them into taking care of the visible mold problems that I've seen, but that's as far as I'm going to get with that.

My apartment is 800 square feet, and the air circulates through other apartment(s?). I'd like to not spend more than about $200 on the air purifier. I realize that a $200 unit probably isn't going to be able to handle a job that large flawlessly, but if I can get some of the pollutants out of the air, I'll be happy. I'm getting ready to start a lot of seeds. If the coir grows thick mold, I'll be very disappointed.

So - can anyone recommend a brand or model?

Am I being completely unrealistic, and maybe I should consider spending a lot more (ouch)? What about filters? Do they make them with permanent filters that you can clean and put back in? Or do you always have to go through the song and dance of who has my replacement filters in stock? Oh I have to order them online and pay shipping? Oh my model has been discontinued and they don't make the filters anymore? . . . ya know?