Isn't the disesal ST just a tarted up Titanium? It does not have the stiffer chassis and suspension and steering does it?
The ST TDCi does have the quicker steering the same as the ST220, and the same suspension geometery - the front arms are different to standard due to the wider track.
The springs and dampers are specific to the ST TDCi - it's not quite as low as the ST220, but IIRC it's 15mm lower than standard.
I've just swapped my ST TDCi for a Mazda 3 2.0 Sport. The comparative silence from the engine at low speeds is deafening!
The TDCi was a great motorway/distance car, but the sharp clutch, diesel clatter and general harshness made it pretty horrible to drive in heavy traffic.
I had the car 7.5 years so I'd got used to it, but the NA petrol is so much more flexible - the ST TDCi had to always be in the correct gear and needed lots of downchanges as it laboured really easily which will quickly trash the DMF.
However, I owned the car from 5 months old to 7.5 years old, 7k to 82k miles. It never let me down. It had a new bumper at about 12 months old due to sagging, which I later had to add screws to to lift it back up, and it had the injectors and the pulley/belt/tensioner replaced under warranty at 38k,
I had to have a new exhaust at 6 years old - be warned the ST TDCi factory exhaust is no longer available from Ford - your options are a different replacement back box from Ford at around £180 where the tailpipe is in the wrong position and you can't see it, or aftermarket - I had a Powerflow stainless steel full system made up for £250. I also had new handbrake cables and a caliper last year which was about £220.
At the point I got rid, the aircon had packed up (suspect the condenser, it dumped all the gas), it needed new front springs (aftermarket ones are not available, £150 a pair from Ford) and I think it was getting ready for another set of injectors and the pulley was vibrating away again.