In the UK the M3W isn't subject to an emissions test after it's delivered so one less thing to worry about.
Not immediately, but after three years the MOT test will assess compliance with EURO 4. And, as I mentioned above, owners of converted, and therefore non-compliant EURO 4 vehicles could be prosecuted if they pay VED for that band or if they sell them as EURO 4. How would a dealer be able to advertise second-hand vehicles without falling foul of Trading Standards? Am I missing something?