Well, for one, you should join NASIOC. Secondly, their stock system is horrible, so if your buying brand new, just get the cheapest system available.
Now, on to speakers. I only know of Impreza for sure, but I'm fairly confident in saying that the front doors are 6.5" mountable, but they might have fitment issues. The depth of play you have to work with the stock mounting depth is roughly 1.5" - 2" at most. For the front speakers, you will have to make a speaker baffle.
As far as pulling the car apart. I can have my entire interior trim, and dash, apart in less than 20 minutes if I have all the right tools available, which is a #2 phillips head, a magnet in case you drop something, and a flathead #3 for some plastic pins.
Remember when you are taking the door panels off, have the windows all the way down, pull the electric controls off first, do NOT take out the center screw in the doorhandle bezel, and be careful pulling it apart.
Anyways, that is advice for the 02+ Imprezas, but being in 3 different Subarus over 5 years, they don't do much different between each vehicle, which makes it a payload much easier to work on and get parts for.
Good luck //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif