BMW X5 3.0D, it'll be getting on a bit, it'll cost lots to run, i.e. to replace the tyres will require you to re-mortgage your house, the cooling system will blow up and it might eat it’s own gearbox as it get over about 125k ...but otherwise it's a nice car, loads of usable space, high driving position so one can lord it over the great unwashed and decent performance ...but wont hurt you too much at the pumps.
On the other hand if you’d be happy in any kind of Peugeot or Vauxhall (VXR's accepted), don’t by a BMW, it isn’t the car for you.
You say all the 5's have galactic mileage, I beg to differ, while they are getting on a bit I have seen a number of tourings with circa 80k mileage, this is not even close to galactic, indeed for the age of the car it's pretty much what you would expect.
Here is a nice example, it's got a pretty naff spec and it's expensive, but it's looks like one of the best example you will find these days, it's for sale only 2 miles from me, I thoguht about looking at it myself, but it wasn't what I was looking for. It has one of the best interior colour combos I have seen in a 5 series too.