diesels don;t need a cat to pass an MOT and they are very easy to pull more power out of.
the ford above is meant to be a very good car, and great to drive. lots of pull, all the mondeo ST trimmings but can do at least 45 to the gallon not a bad compromise.
as for noise i've heard alot of these new diesels can sound quite fruity, and alot less agrigultural. i think they are better for sound if they have more than 4 cylinders.
i don't know if the modeo has one, but if it does a DPF removal is also worth doing, they kill MPG and with them gone there is more power and better MPG
(DPF = diesel particle filter (its designed to stop smoke from the exhaust it filters it out, and then on a long drive the engine purges the filter with fuel and burns it off)) again not needed to pass an MOT. and when they break which they often do they aren't cheap to replace, but a DPF removal is alot cheaper.