Any reasonably modern diesel engine should last at least a quarter of a million miles.
I'd expect to get 60mpg from a 1400cc diesel engine, unless you're going to have really heavy loads in it.
Unless you're likely to be carrying gold ingots, you probably don't need to worry too much about the payload. (The general rule for a Fiesta is "if it will physically fit into the van, then it's not too heavy for it").
You'll have no problem driving up to maximum speed limit with a diesel Fiesta. (Because it's 'car derived' you'll be subject to the same speed limits as when driving a car. If you'd have gone for something like a Transit, lower speed limits would have applied on some roads).
There's no such thing as cheap courier insurance but this is one of the best-known places to try:
http://www.gladiator.co.uk/
Chris