If you're only doing about 25 miles a day and don't need to use the van for long journeys at all, another option would be an early electric van. 2015 examples of both the Nissan E-NV200 and the Renault Kangoo ZE are within budget.

These are effectively a van body with the electric running gear from a similarly aged Nissan Leaf or Renault Zoe. The range will be terrible - probably no more than 75 miles on a good day, but for purely local use could be a cheap to run option provided you can charge at home overnight.

We have a number of one man/woman band dog walking outfits locally and one business that has flourished over the last 15 years or so. All seems to use diesel Berlingos with the successful business having a fleet of them in a non standard colour and mostly private plates related to the name of the company, so difficult to immediately spot the age, but they are not purchased new. Probably mostly 5-10 years old. Can't imagine they'd stick with them if they were that unreliable.

Same with Euro Car parts, they use Berlingos or Pug Partners. Ditto Post Office.

Re Royal Mail. Most of the postie delivery vehicles locally are 10 - 14 plate Peugeot Partners with a couple of slightly older VX Combos. Generally a bit tatty but generally around 50k miles. We're about 3 miles from the delivery office, the van (with two posties) parks up whilst the they split up to do a portion of the round, then it moves on 1/4 mile and they do the same again. I doubt the van ever warms up, especially in winter. Not generally what's recommended use for a diesel.

The two dog walkers in our area run a Pug 206 estate and Skoda Superb estate respectively, the vehicles are now effectively two seaters, so long as you avoid the rather silly fashion type estates which are only glorified hatchbacks and look for estates with more squared off rear section once the rear seats are lowered (or even removed completely) they are suprisingly spacious.

Worth a search for ex police vehicles on ebay, you often find ex dog section estate cars being sold cheaply, already kitted out with well made crates, maybe possible to sub divide those crates to allow 3 or even 4 various size pooches to be carried.

We have a Vauxhall Astravan 55 Reg with the Isuzu diesel engine and no DPF. Simply brilliant on fuel and well known for long life. Built in the Opel factory in Germany so excellent build quality.

These fetch less than £2k at that age so depreciation is not a problem