Any engine will need maintenance and replacement of parts, so to me "worn out" means worn bores and crankshaft bearings.
Unless you've driven a few of these vans it's unlikely you'd know if the engine was good or not just by driving it. With a stock tuned engine they're slow, but how slow is slow? Hauling a 4400lb brick with only 85hp is never going to be fast.
A compression test would be the best way to detect bore wear. At sea level* a new engine has 440psi, the lower limit is 320psi, with no more than 20psi difference between any two cylinders. *(If you're above sea level then adjust the pressure to suit,
as we eventually figured out in this thread. In my defence New Zealand doesn't have altitude, so I have no experience with it.)
Without a compression test, oil leaks are the main tell of a tired engine: 4D56's are heavy breathers, with lots of blowby. As the bores wear the blowby becomes excessive, overcomes the breather and pressurises the crankcase, which leads to oil leaks. That said, these engines can be leaky: there's a
rubber half-moon bung at the back of the head that hardens with age, and a side cover by the injection pump that leaks when its gasket hardens, and leaks even more if someone over-torques the bolts in an attempt to stop the original leak. Front and rear main seals are the ones to pay attention to.
There's
a "new delica" mechanical checklist here which will give you an idea of things to look out for, and things to consider replacing as preventative maintenance. It might be a bit overboard. I'm conservative/cautious when I buy a vehicle; I like to change all the fluids and filters, replace *everything* rubber (coolant, oil feeds, brake lines, suspension bushes, sometimes even drivetrain mounts), and if the coolant is dirty I'll replace the radiator too. Because I do not trust anyone else to maintain an engine properly.
