1.6 more headroom for motorway's, but make sure its over 100bhp. I know you're not looking for performance, just somthing that is reliable and get's you from a-b.
Maybe summit Jap? A Honda Accord would be very nice, but if you are wanting to stay with the blue oval, probably better off with a Focus given their lower pricing nowadays.
You can always pick up pre-converted Vectra's and the like if you were ever to make the jump; but as always, you're number one priority is reliability due to the nature of your missus's work, so it is important the engine doesnt miss a beat.
Hence I would go for Jap for petrol. Make sure whatever it is, it come's with the relevant certification's, as I think some insurance companies will penalise a non-certified installation.
The price of diesel is scandalous atm, at it is getting to the point where it is no longer any cheaper to run a derv then a petrol because the extra milage you make per gallon, you are paying upward of 40p for.