As said above, you need to be specific about the size of load you want to carry.

You also need to be aware of maximum axle loads.

The 'light chassis' versions of the Fiat Ducato/Citroen Relay/Peugeot Boxer/Vauxhall whatever typically have a max rear axle load of 2000kg. The 'heavy chassis' versions have a typical max rear axle load of 2400kg.

Budget - £5000

A big van at that price is probably going to be on its last legs and/or done a huge mileage. Bottom line is that it will be a massive gamble, so how lucky do you feel?.

Petrol or Diesel - doesn't matter

You might find an LPG option, otherwise it will be diesel.

Manual/auto - Any

Because of the likely condition/mileage of whatever van you look at, an auto (were ne to crop up) is probably unwise.

Think we'll be using a load of 1000kg - 2000kg.

You are not going to get a 3.5t van able to (legally) carry a 2000kg payload.

Good luck, you'll need it!