You'll struggle to get anything over 220bhp with a smic, seriously go for a fmic.
I'd also add a downpipe to your list, as it's by far the most restrictive part of the exhaust system. What about an upgraded tip? Forge diverter valve? Panel filter? Is the car mechanically sound at the moment? Before you start upping the power you need to make sure there are no existing faults.
Even at the top end of your budget you're going to struggle. It'll need a custom map which will be a minimum of £300, even a 3" eBay exhaust will set you back £200. That leaves you £200 for a turbo, intercooler and any other bits...