A car of that age shouldn't have any real problems and the high mileage shouldn't be an issue as it's mostly all probably motorway miles. If it's been serviced in line with Fords recommendations it should be ok. I assume the garage is offering a decent warranty? If not, you should haggle for a year.
I was lent a 1.8 TDCi when my own car was in for a service. Compared to the 2.0 it's a bit sluggish below 2000 rpm but picks up nicely after that. The 5 speed gearbox isn't as nice to use as the 6 speeder in the 2.0, it's a bit notchy but no worse than my old Picasso.
I filled my car up today and over the last 228 miles it took 22 litres. That works out to 47 MPG although the trip computer was saying 43.5! I had to squeeze the last Half litre in, and if I hadn't it would have been over 48.
I would think the 1.8 would be about the same, probably a little better in similar usage, and driven with a bit more reserve than I usually exercise, I would think an average of 50 would be easily achievable. There are many threads about this on this forum if you look.
There are others who are more savvy than me about common CMax faults, Suffice to say that mine hasn't missed a beat in the time I've had it so I can't really comment on what goes wrong!
Just check the usual stuff, uneven tyre wear, etc. Cmaxes seem to be a bit heavy on tyres so make sure there's plenty of life in them. I put a set of New Cooper Weathermaster tyres on mine in November and the front's are nearly half worn after only about 3000 miles (Max) My son has the same tyres on his Xsara 2.0HDi and his are less worn after 2 winters and many more miles! They started off at 7.5 mm, the fronts are just over 4mm and the rears are 6.8!
2008 2.0 TDCi Zetec