vans are always a difficult one mate, so you should pay more attention to the previous owner than the mileage... this is why, in theory a diesel should do 200-000 miles, however, if the van has been driven by a big hairy builder who smashes it all over i doubt it would se 90, however an ex parcel delivery van oild changed all the time etc will do loads more.
pay attention to who your buying from not whats on the clock, even a newish van can give you expensive problems when its not uncommon to pick up an 98 escort and run it for years