I have only ever made small parts in carbon before but am thinking about making a bonnet. One quick question, is it worth shelling out for a vacuum infusion setup, or can I just lay carbon with a brush as normal? Are the results really that much better?

Yes infusion is the way to go,I would recommend you make a few smaller parts to develop your skills and study the EC tutorial https://www.easycomposites.co.uk/learning/how-to-make-a-mould-for-a-carbon-fibre-bonnet