Just my 2 pence worth but....
Contrary to popular belief, Police Cars are not maintained regardless of cost, they are maintained according to how much (cut by HMG) budget each individual force is able to pass on to its maintenance / garage etc
Computer Service printouts are all well and good but they'll probably be the same intervals as VX (20K oil change on a hard driven car anyone?), tyres should be a good quality but interiors may well be trashed and everyone who buys one will be looking for the same trim as you are.
Wiring in these cars is usually severely hacked about and often decommisioned poorly.
Now if you get the car for very little then these things are irritations that you can sort over time.
I'm not criticising, I'm just purely trying to point out that these cars have generally led a hard life and that must be taken into consideration when you are spending your own hard earned money.
As a project for show use then it's not such a problem, as a daily driver you have to consider that all the time, effort and money required to keep it running....