Although I've been involved with engineering of motor cars, this doesn't mean everything I say about them is right, but perhaps I know more than some.
Every mass-produced car made has seen a reduction in quality over the last 10 years or so - some were pretty poor to start with, too. Combine that with increasing complexity and performance and you've a huge potential for problems. I'd avoid all diesels post-2004 unless you rack up a mega mileage or tow heavy trailers a lot. They're exceedingly costly to fix and unlike old diesels, do routinely go wrong at vast expense, especially Fords, Vauxhalls and Renaults. (I wait for the "mine never has" indignation!). People will spend thousands extra just so they can tell their mates their car does 55mpg+, then have repair bills which run well into four figures for something which once upon a time would have cost £40, but which never needed replacing anyway.
Police vehicles are not all well maintained, it's a bit of a lottery. Many 'panda' cars can be quite worn out and relatively neglected, I've heard of traffic cop cars which are bad buys too - less maintenance than there used to be and often no longer in-house.
Skoda Octavias are well-sized, and better built than the VWs they're based on. The original (Mk1) is a very good choice, they were made up to 2007ish I think. Up to 2004 they had bomb-proof diesels (pre-PD) and often average 60mpg+, lasting well over 250,000 miles with little or nothing going wrong - provided they're serviced properly.
A Dacia Duster is a good buy - better than spending £6-8k on something
second hand, if it suits. They're decent cars, better than many at twice the cost. I think Renault have sorted the diesel probs.
Whatever you choose, don't stretch the oil intervals much beyond 8,000 miles or a year - manufacturers find that in general engines outlast cars, so try to extend oil changes to daft extremes, it looks good for fleet purchasers. And change gearbox oil every 100,000, even if not specified.
I wouldn't have a car with less than 150k on the clock - too much money tied up unnecessarily - and all the teething problems will be properly sorted. Servicing is the key to longevity and reliability, if you start off with something decent.