Basic mods would be induction kit (Gen II) remap (Bluefin or Dreamscience) and maybe an exhaust (Mongoose, Blueflame, etc) - can always go for a decat too.

tbh those mods shouldnt affect mpg that much - really depends more on how you drive the car depending on what mpg you will get (same as most cars)

Bear in mind though if you dont plan to mod much just stick with the basics otherwise it can be costly - i bought mine nearly 2 years ago with an exhaust, filter, etc and said "i'll keep it like that" and have spent far more £££ than i expected and have one of the more powerful and uniquely modded FRS in the UK

Really all depends how far you want to go Ross