Ok,
I have one of the older Kenwood CD/MP3 units and it works fine.
The 2 main things to look out for (IMHO) are:
1) Speed of access to tracks.
2) Navigation
My (old) kenwood takes about 5 seconds to read a MP3 disk when I insert it, i'm sure the newer models have improved a great deal.
Navigating through the folders/songs can be a bit of a pain.
My advice would be to burn a MP3 cd and take it down to Halfords, try out a bunch of head units, decide what you like (speed/naviagtaion) and then order it online!
R