Just wondering why this one is cheap compared to others that don't have the 2.5 engine
Because generally the buyers of a large family car are more interested in lower running costs than performance (or as you discovered "perceived" running costs - i.e. doesn't really cost much more in Insurance, or VED, and unless you are doing huge amount of miles isn't going to cost much more in fuel).
Also:
Remap will see you over 250bhp e.g.
http://www.mybluefin.co.uk/search?make=9&model=61&fueltype=3&variant=1847

