I think it is best to check the train schedule and rates on:
http://horarios.renfe.es/hir/ingles.html.

The fastest trains are the AVE, the Talgo and the Euromed. It will depend on your departure location.

If you go by train it will at least take you 8 hours to get from the south to Tarragona (closest city to Salou). If you are close to Seville, then take the AVE to Tarragona. You can take a free shuttle bus to Salou from there.

By car it will also take you about 8 to 10 hours. You can hire a BMW 2.0 for about 450€ getting 2000 kilometres free. A VW golf will cost you around 250€ with the same free mileage. Spain has very good highways and it is relatively easy to go from the south to the north.

Also check http://www.clickair.com or http://www.veuling.com for cheap tickets from Malaga or Seville to Barcelona. If you speak Spanish try INVALID URL it's a Spanish comparison site. I flew from Barcelona to Malaga and back a few weeks ago and it only cost me 180€. It was an hour and a half trip.

I would not recommend the coach. It will take you ages and it is very frustrating to find the right once, especially when you have to change coach several times.

Thank you very much for your response. After checking a route planner, I realise it is a lot futher than I first thought, so I don't think we are going to manage that.