I opted for a Pioneer SPH-10BT as headunit along with a Single DIN adapter to fit the dash. That way I can use my mobile for music and navigation and don't have to bother about physical media. As much as I remember the stock dash cut-out is 1.5-DIN or similar. Not too many units available at that size...
Concerning the speakers I don't think you'd get a noteworthy SMACK out of the stock locations. I haven't gotten into a speaker upgrade yet but have a 2-way system (custom/separate kickpanels in legroom and tweets in dash) for the fronts in mind. For the rears I think deleting them and putting a sub box in the trunk would be best. Sizes may vary depending on level of desired SMACKiness ;-)