Speaker Recommendations and 6 disk player/Radio replacement

Hi, I have two knackered speakers on my 2005 freelander 5 door, both on the front doors. I was wondering if anyone could give me some advice what are the best replacement speakers and if i need a new pre-amp to get better quality.

Also, has anyone replaced the 6 disk/radio unit in the car before? I was going to put something like a Kenwood Double Din Sat Nav/BT Unit in. Just wondered if this was possible and if it is an easy fix?

Cheers