Hi Jay,
Don’t know about changing over to left hand drive. I know it’s not required for cars but it may well be different for vans.
The price of re-registering a vehicle depends on a number of factors, including engine size, fuel type, age and estimated current value. If you use an agent to do the re-registering they will have a guide issued by the authorities and will be able to give you a quote.
I used an agent in San Pedro de Alcantara, Malaga, to do the whole thing for me. He only charged 90? to do the paperwork. The re-registration cost 1,100? for a 2 litre diesel Citroen Picasso which is 5 years old. Still worked out cheaper that selling the car in the UK and buying another one here.
Be careful, there are many agents who are charging a lot more for their services. Ask to see the charge in the official handbook so you know what you are actually paying them for their time. A friend paid more than 700? just for the agent.
Good luck.