The concern would be the electrics.
I don't know anything about the Proace but the Ford / VW Galaxy / Sharan used to have electrical issues if water found its way into the boot.
Although being Toyota they've probably thought of that.
They are exactly the same vehicle as a Citroen. Just a couple of cosmetic changes.
The people carrier version usually has a lower payload as Dazmond pointed out.
Don't get hung up on vat Tristan. Vat is added to every vehicle, car or commercial. You can claim vat back if you are vat registered. If you aren't then you claim it against tax although it's not quite the same as getting it all back. Vat is just 'hidden' in the price of a car.
Being a people carrier there is no provision for a bulkhead. Also due to side windows, having pole carriers becomes more difficult.
Pros are the people carriers aren't speed limit restricted on single carriage and dual carriage highways as vans are.