Its not so much that they have a mileage where they just stop working, it's all about how they've been used and looked after. There are people with TDCi mondeos on 250k miles and upwards - most people who have been in a taxi will have seen one of these
The 2 big potential problems are DMF failure (dual mass flywheel) and injector failure. The rest of the engine, and the car in general is pretty solid - I had mine 2 years and the only non service cost was a new DMF and clutch at £650. A fairly expensive repair, but in a cost per month term it wasnt bad!
DMF's get killed by poor driving mostly. You know how most cars will judder if you use too high a gear? The Mondeo doesnt do that nearly as much, I believe the DMF smooths that out. Due to the nature of the engine, people are much more inclined to use a high gear so that can end up knackering the DMF. Cars that have done a lot of stop-start work around town or that have been driven by people who cant change gear smoothly will also be more prone to failure. Rattly idle, sqeauling clutch or trouble starting are the symtoms.
Injectors are the second thing. To be honest, I'd be surprised to see a car that passes 100k without at least 1 needing replaced. Again, lack of care and poor driving provokes the issue. Cars that get run down to nearly empty and driven by people who redline it a lot (a pointess exercise in these) will suffer more than ones who get a nice clean full tank and driven normally. You're talking about £150~£200 per injector, less if you get the existing ones refurbished. Ford will always say to replace all 4, leaving people with a large 4 figure bill, but a diesel specialist can diagnose them and replace/refurb only what's necessary. Look for smoke on a cold start, excessive smoke under hard acelleration or the car going into limp home mode with the glowplug light flashing.
You want to be looking for a 4~5 year old car with a good solid history and 1 or 2 previous owners. Don't be put off if you see reciepts for stuff like DMF replacements - the DMF in particular is a more robust part now, so a replacement could be a good thing!
In terms of mileage, you want one that's had an easy life - so somewhere between 40 to 80k would be in the right ballpark. Not too low to be an abused car around town, not moon mileage. If these are looked after, there's really nothing to worry about. You only hear so many complaints because there are so many out there, you've just got to look at the number you see on a daily basis to realise they aren't the timebomb some would have you believe