The depth should not be an issue, if your reason for a Volvo replacement is only the possible ease of fitting, then I would suggest your money would go much further with a quality after market system from Pioneer, Alpine, Sony, etc.
Depending on your budget, Halfords do quite a good range with free fitting, better quality, better audio capabilities and with the ability to add amps, subs and additional toys later on.
Have a good think about the pro's and con's before you make the final decision, I have never known a car manufacturers system to have been any good unless of course it was designed by Bose, Mark Levinson, Naim etc,