If you want 350 expect to spend a good chunk of cash. To achieve that you'll need a bigger turbo, decat exhaust, new intercooler, fuel pump, possibly injectors(unsure for your model), remap. 350hp is also considered the limit of a 5 speed gearbox before it goes pop. Internals may also start to struggle if it's all still standard. With all that power you're gonna need to stop quicker and get round the corners so consider brakes and handling mods alongside.
Starting out on a standard car it's easy to get reasonable gains in power per pound but the higher you go the less hp/£. Someone with better knowledge of your model may pop along and offer better advice than I can.