I would never buy one and this is my backed up and educated opinion from experience.
-The engine is built with very weak internals, Mazda is known for doing some things well in a particular car and then neglecting other things. There have been a significant amount of engine issues associated with the lack of attention in this area.
-The stock turbo is maxed out, once you get your bolt ons done the stock turbo is begging for its life and though it will function you will need to upgrade the turbo for anything above the basics
-Transmission problems: There have been a significant amount of issues reported regarding syncro failures
-These problems have been known to happen in stock or modified cars. So modification is NOT to blame. People have blown motors by being on the highway in 6th gear and boosted half way, let off and pshhhhhh.
Mazda in many cases has made a good car on paper, and in initial quality and skimped hard on what needed to be made sure was top notch to begin with. The MS3 has horrible ergonomics and the power straight up DIES above 5,000 RPMS. The handling is not as spectacular as it is made out to be, 600ft skid pad hits about .87g, not bad but you will have traction problems as well in anything but dry pavement. Not exactly torque steer but there won't be much fun unless you are on the dry.
I personally do NOT like these cars, there are others that are better for the money. Magazines blow this thing up as if it's the best thing for the price ever built but it is not, not at all. I have driven many my self, and know people with stock and modified MS3's and I would like to formally say stay away.
If you want something that won't give you those issues, grip and is in fact faster in the straights as an 08+ STi, responds to modifications just as well and for around the same price with the same utility NEW then buy one of these:
However if you want to go used there are a million cars I'd rather have.