From the spec it's at now I would start with a large capacity intercooler and go from there.

You will want better brakes as RST brakes are frankly crap. And a decent suspension kit to tighten the chassis up.

There is loads you can do it's just a case of how far you want to take it.

Interior wise you can add the leather gearlever gaitor from a Mk3.5 Fiesta and fit a good quality gearknob and steering wheel. Make sthe interior so much nicer.

You can also add the add ons that are missing like the trip computer (usually more trouble than it's worth), variable intermitant wipe and the extended handbrake console (if not already present). The car looks like my S2 where in it has 90 spec seats but not much else. Least that one has teh spoiler I had to add all of the 90 spec to mine