I can recommend the one in Pauls post however you would have to buy a seperate nav unit to plug into it, although there is apparently an iPhone app that connects to the JVC but I haven't tried it so can't comment.
Unless you do as Level 42 did and try the 'Chinese Way' you will have to spend a fair chunk of £ to get a DD with all the bells and whistles and Nav too.... Nothing south of £500 new I wouldn't say
Some of the newer Pioneer AVICs or Alpine units are in excess of £1,000