When Buying An Air Purifier, What Should I Look For?

I am wanting to purchase an air purifier for my apartment, which is approximately 700 square feet. I am wanting to eliminate light cigarette odor, pollen, dust, and perhaps pet dander in the future.
What should I look for when purchasing a purifier? Are their particular brands, features, styles, designs, etc, that I should look for?
Any product recommendations? I am looking for a price range of $75-$150 (USD)
Thanks for the advice!