Kenwood KDC 3047 is around £80.00, AM/FM, CD/MP3?WMA playback, aux in, 4 x 45 watts. You'll need a fascia adaptor, loom adaptor and antenna adaptor all of which will depend on the age of the car but your local dealer will advise on that.
You should get the whole thing for about £110-£120 if you're fitting it yourself.
you can also purchase a fascia that slots around the stereo to cover the gap left by the original one coming out.
if you stick with kenwood and pioneer you can't go wrong and can pick some decent ones up for cheap.
local to me is halfords and bass junkies, try looking on bass junkies web page has they are always doing weekly specials with stereo systems, also they'll tell you exactly what else you need to install it.