Go to a specialist installer and get their advice.
I reckon a decent amplifier and upgraded speakers added to the stock headunit would do the trick.
Alternatively, there is a reasonably good equaliser on the headunit, it's kind of hidden in the menu.
When I eventually found it I was able to create to a custom setting that was a vast improvement over any of the pre-sets or the flat EQ.
Before this I was thinking of an upgrade like I suggested above, now I'm not too bothered and am quite happy with the sound quality.
I think others on here have found the same.
Once you get hold of the car, check out the manual and have a play with the EQ and see if you can get acceptable results.
Edit: I see GCDBONES has beaten me to it!