Air purifiers or Air conditioners to prevent traffic pollution coming into the house?

1) Depends on the purifier capacity; they generally list the size of room they are designed to keep clean as they recirculate the air in it. Generally, for a whole house, you would have one in each major room. I just keep one in my bedroom so I sleep in cleaner air.

2) Sounds like the air isn’t all that fresh, really. Without AC though it will really get stale/warm if completely closed. Maybe open the windows at nights, when it’s cooler, lower traffic/pollutants?

3) It depends on the purifiers. There are ones that filter using conventional filters like the ones in built-in air conditioners; you would buy replacements regularly as they fill with dust and grime. It sounds like you are talking about the ionic ones though, where you clean off the metal plates that attract the dust and other pollutants through a charge as the air passes through, instead of pulling the air through a filter.

Ionic air purifiers are cheaper to run since you don’t have to replace filters all the time (though you do still have to maintain them) — but they do produce ozone. While ozone is good in the upper atmosphere, it isn’t so good at ground level; it can worsen respiratory issues like asthma and damage the lungs. I’ve read some reports that they don’t work as well as conventional filter units at purifying air, either (Sharper Image lost a big lawsuit not long ago over this — they were the ones who first pushed these in the US).