You can't just put bigger discs on. Ebay has new 294mm discs/calipers/pads for about £200. On top of that you would need new caliper carriers, probably £20-£30 used for a pair. The protection plate (what I call the dust shield) is the same so that doesn't need changing. There is a world of possible options, I would go for better pads and discs and reconditioned calipers. For what it is worth I think it is worth doing better brakes gives you increased confidence.