I totally agree with 'kyleb.' An exhaust fan is the surefire way to go. From the research I've done, an suggestons I've received (I plan on smething similar soon) Penn Barry seems to be the best exhaust fan for the money, performance and reliability.
HERE is their site. Poke around on there, lots of options, whether it be a ceiling, wall or inline fan.
If you are set on an air cleaner/purifier, I have four air purifiers throughout my place:
(1) Blueair 501 - expensive ~$500, but really gets the job done, great for large rooms, quiet until it gets to the highest speed then it's like a jet taking off - 601 model is $100 more but no difference except side intake vents.
(1) Kenmore Evirosense (larger model) - ~$250, got mine for $200 I think, most Sears' have them on sale. Great unit for medium sized rooms very quiet. My preferred purifier for bedroom/office something around 16x16. Also an Ionizer.
(2) Neoair Enviro Plus - ~$300, I got two for $500 on some website. Alright purifier, performes best in small bedrooms, something 12x12. I use
both in one room, and together they perform slightly better than the Kenmore. Also an Ionizer, plus it has a place to put scented oils or those little scent-cans you see at carwashes.
The Kenmore has the best design, but according to the Blueair's packaging, it won a Swedish Design Award, personally I think it's ugly, and it's big, about 26"x10"x16", that's going off memory. For a loft, even a den, which will be a smoking lounge, I think an exhaust fan would work best, but the money involved is obviously much greater than air cleaners/purifiers. Getting the smell of smoke out of a room is difficult, an ozone purifier, like a Csonka is a good idea for after you smoke. The company says you can run it all year, but I wouldn't do that, after you smoke for an hour or two is fine. One of the BOTL sponsors,
www.cigarmony.com, sells the Csonka ozone purifiers.
These are just my opinions, good luck!