Like several other members of this forum, I had a bit of experience driving emergency service vehicles, of various makes, petrol and diesel.
Funnily enough, although they get a hard life, in my experience, the engines generally seemed to be really reliable even past 150K miles, a T6 used for organ transport might have had an easier life than most emergency service vehicles.
I would look carefully for damage caused by kerbs and speed humps and at the seats, they get a hard time. Obviously there will be holes where you don't want them and wiring that makes no sense, but that should be reflected in the price. It will almost certainly have been well serviced and any repairs will have been done properly.
Very importantly, check if holes were drilled in the roof for blue lights and wiring, if they were they need to be properly filled and sealed, I've seen ex Police vans go to auction with gaffer tape over the holes in the roof, which on a van can't easily be seen unless you are tall.
Like any used vehicle, it could be a good buy - if the price is right.