High milage diesels scare me, whilst older ones were more reliable than their petrol counterparts, buying into the modern ones at a high milage just seems to me like paying a premium for the previous owners savings on mpg.
70,000mile petrol Vs 110,000 mile diesel focus's at that sort of price, I'd be looking at the petrol ones, the engines are simpler and should last the same number of miles, everything else has 50% less wear and there are less big ticket items to go pop. Conversely you probablys till need to be unlucky to actualy have any breakdown.
I've a C-max 1.6 petrol, does 40mpg on DC/motorway at DC/motorway speeds, 42 if you treat 70 as a limit rather than a target, guess the brick aerodynamics aren't heloping there as it will do a ridiculous mpg (65+) if you have to stick to 50mph after puncturing and fitting the spacesaver! So a normal Focus should be 42-45mpg easily.
Also Saab 9-5 2.2 TiD - pretty bulletproof engine plus relatively little else that goes wrong. Beware Saab 9-3 Sportwagons with 1.9 TiD engine - much less reliable and cars generally inferior build quality - GM's legacy.
GF's brother in law has a SAAB with the 2.2 diesel engine. The Engines a chain driven cam Isuzu. Not as economical as a modern diesel, but it's an already reliable engine from a small truck, in a car, so gloriously understressed and never likley to pop as a result!