Hi, I currently own a 2004 Mazda 3s. I've owned it for 2 years and it has been a blast! Coincidentally, my car has a similar number of miles as the one you are looking at (94k). Some things that you should look out for are blown struts, worn out heat shield under the car and the tires. Mazda 3's are notorious for eating tires so I would recommend rotating yours every 2-3 oil changes at least. Check for a "wooping" noise when cornering. This could either indicate ill worn tires or bad wheel bearings. Other than that, I really can't think of anything else. All the issues I have addressed are not an immediate safety risk and are relatively inexpensive repairs. Another thing to look out for (if you do end up getting the car) is the internal thermostat. It has been known to fail in our cars and will set off your check engine light, but is hardly an issue.
Best of luck with your new Mazda!