Before I buy.

I have decided that I want to buy a Subaru. I want an '03-'07 impreza wagon (manual). I've done a reasonable amount of research about the engine and transmission. I was wondering if there are some notorious key points I should look at when I'm test driving/checking out prospective vehicles. Also, should i be weary of any after market electronics or parts? Any and all advice is welcome.

Edit: thanks for all of the info and resources guys. I've decided to cool my jets a bit and get my current car in proper selling order. I'll resume my search around mid February. In the meanwhile, I think it'd be beneficial for me to learn as much about the specific model I want. If at the very least, I can purchase with a proper amount of scrutiny and be confident in the purchase.