Mercedes W211 Buying

[meesterb]

I am thinking of buying a W211 diesel auto probably about 2004 vintage. I would be really interested to know what pistonhead experts know about this model of E-Class and in particular if there are any areas which should be watched when buying second-hand. I know these cars are meant to be tough but as most cars of this vintage will have >100k on them by now i would be interested to understand how reliable they are i.e. engines/gearboxes/turbos/electrics etc. Not sure which model of CDI is the best all round balance of power and economy, maybe the 270? Would also be interested what kind of MPG one can expect. Lots of questions i know but i would really appreciate your feedback and advice. Many thanks!