Greetings and welcome to the club!
Buying a used anything could be risky, especially high performance cars as they are more like to get beat on. I would have any car checked over by a good auto technician before buying.
There have been issues with the struts failing on the 2004 - I know there are threads devoted to this subject. Many never have problems.
AWD is only as good as the tires and the driver. The OEM tires would be awful in the snow. At the very least all season tires will be needed - dedicated winter only tires are best.
Mileage varies quite a bit but I think I am safe to say from 17 - 22 mpg.
Have a look here: http://www.clubwrx.net/forums/car-purchasing-forum/ for more info. Happy hunting.