I just replaced the HU on my 2013 wrx, it seriously is a major downfall of the audio system in the car. The stock speakers aren't that bad to be honest much better than the ones that are included in the 2007 sti that my friend has. I went after an Alpine unit single din, i despise touch screens of all kinds and using one in a car while driving in traffic would probably cause terrible road rage.
From a respectable sound engineer "You can make 10$ walmart speakers sound good with the proper equalization" consider that when thinking about a HU get something with an equalizer and it can mask cheaper speakers only down side is you can't make an amplifier more powerful with an EQ.
Good sources of info that i've found.
You will need adapters for the speakers there are some good sources for those on the previous links. Also consider sound deadenign for your doors it will make everything sound better, even driving with the stereo off.