Being fairly familiar with the modern stuff - they are still relatively heavy - and surprisingly cost prohibitive - yes, there are some cool small 4's and even some v6's and straight 6's that put out good power but they are EXPENSIVE for the power you get out of them - if you're goal is power you'll be better off with an older merc - they are just so simple with not as many parts - the wiring on the newer cars is tough and then if you want to push the power up you've got custom tuning - which is getting better and there are good people in the business but it isn't 30+ years of time like the older mercs have had to get their pumps & injectors perfect and proven.
now if you wanted to not rock a hood you can pick up the touareg v10's for a relative song for what they are - 300+ horse & 550lb/ft stock is a lot more than most stateside diesels you'll find and can of course be easily chipped up a fair amount.
