I am no expert, (so pick me up boys and girls) but basic advice worldwide seems to be:
Cat back exhaust.
If you want to go further, most go
Cold Air Intake
65mm throttle body
Then we get into cams, underdrive pulleys, adjustable cam pulleys, headers, superchargers and the whole catastrophe.
Chips are a big debateable issue. Most seem to suggest that until you have substantial mods, they add little if any. Powerchip claims 96kw goes to 107kw with their 98 Gold chip, but have yet to hear of anyone getting that.
I was going to add a Unichip piggyback but was tlod by someone who should know not to bother.
best site to start are Focussports. Focus Central, Focaljet, and FFOC (Ford Focus Owners Club) in UK (Aussie cars are Euro spec not US, but very similar).
There is a company in Sydney www.infocusmotorsports.com.au
(I think) that caters to Australian Focus Owners but IMO their prices are WAY over the top (ST 170 lower grille $220....................$85 bucks from any Ford dealer!!!!!!)
" Performance Ford" Australia New Zealand Volume 4 Edition 3 (a few months ago) has an article on a Focus CL 1.8 where the owner Kirsten Dally claims 133kW after a big series of mods.
I'm sure someone else will give a better viewpoint than mine, but hopefully this is an OK intro.
Everyone has views on which brands for filters, exhausts etc which no doubt they'll let you know ( I have a stainless steel cat back exhaust from Herrod Motorsport which costs a bomb but I'm happy with) "Zetec" has a catback for substantially less money from an exhaust specialist in Sydney that he swears by
Good luck. have Fun