We've got 2 VW T4 Transporters. Good workhorses, spares can be pricey or reasonable, depends what you need. Resale values are high and they are bought as surf vans or converted to campervans. We also have experience of Vauxhall Midi - ancient now so going dirt cheap. Will carry the weight, Isusu engine is totally reliable, body rots away. Can't fault them for the money. Nissan Cargo Vanette totally reliable, bit juicy, body doesn't rot quite so much but great value for money.
In conclusion the VW is good, but for window cleaning a second hand Japanese van will prove reliable, do all the work you want and then you can scrap it for a bit of money.
Hope this helps.
Chris