Regarding issues with the PowerShift DCT in Mondeo, it was only vehicles manufactured in the first few years (late MB and early MC models, approx 2010 to 2012) that had any kind of pattern of common failures. Those tended to have an electronic sensor that could go faulty and cause the gearbox to lose its mind (resulting in jerking/baulking on shifts, throwing DTCs and potentially stranding you). After 2012 a revision was made and that issue was basically resolved; by the time we get to the MD Mondeo in 2015 it is basically a proven reliable unit. Unfortunately it gets tarnished by carrying the same name as the notorious dry-clutch DCT unit that caused so much woe for Focus and Fiesta owners.
As an early model Powershift car would be pushing over a decade old now, I suppose you'd be looking at a MD or at most late MC model. In which case you can expect the gearbox to be quite sorted and not likely to give you any unexpected issues, assuming it's been serviced correctly and looked after through it's previous life.
The DCT does come with a four-year/60Kkm service interval for changing the gearbox fluid, which is absolutely essential to observe. Said fluid does run a bit pricy, which tends to dampen any savings you might realise from fuel economy over a torque converter. Repairs, if they are needed, tend to be very expensive, although as I've inferred we don't really hear of them going wrong any more.
If you're looking at a wagon, the predominant drivetrain will be the 2.0L Duratorq TDCi turbo-diesel paired with the 'wet' PowerShift DCT. A petrol powerplant in the wagon was only available in the base model (being a 2.3L Duratec NA in the MC LX, or 2.0L EcoBoost turbo in the MD Ambiente), paired with the SelectShift torque converter transmission. Wagons in Australia missed out on the EcoBoost turbo petrol available in the hatchback for the mid and high-range trims (Zetec/Trend and Titanium).
IMO, the diesel and DCT is a very good driving package, but
some people say it's just not a combination for them. If that's you, then you'll have to play off whether and how much you want the interesting tech (infotainment and safety assists), versus the option of the petrol engine and no DCT in the lowest grade.