You don't NEED any of it, you can just have a sports car and take it to Niki for Stage 2 mapping as it is. All of the mods mentioned are to extract maximum performance - just be honest when filling in the spec sheet
MPG when pootling around will be no different really, as the cars run at Lambda 1 with maximum timing for MPG/efficiency anyway, but obviously the first few weeks of honing about won't be wallet friendly