Hey mate, Im looking ate getting some coil overs also although im not 100% at this stuff as of yet. Although I spoke with some of my friends and they said coil overs will make your car 'Stance" out. Bad play on words, but you get the idea. They suggested to get some Tein coil overs although its really up to what you want to spend on them. I found these for around 400-600ish AUD$
http://performance.fulcrumsuspensions.com.au/tein-suspension-for/1998.10-2005.07-lexus-is200-gxe10-rs200,-z-edition,-l-edition/SKY06-M1B00
Let me know what you think.
Hope this helps.
Shackz_