Love ST200s, but I'm biased.
From my experiance of my similar ST24, I got good power gains from K+N cone filter, some peeps worry about heat soak, but I fitted couple cold air feeds and had no issues.

Also, I removed main cat, before MOTs got bit picky, and got more gains and car still passed MOT emissions, as this V6 also has 2 pre-cats as well.
Those 2 mods gave good bhp gains for very little money.

Also, if you are going to track it, I would look into a front strut brace, I fitted one and afterwards it cornered like it on rails and squeaks and groans from the suspension disappeared.

Also, check your springs, I had a front spring break, common on these, and considering what you intend do with it and cars age.

Also, check your imrc box is working, they sit under plastic cover at front engine and the electronics get fried due to the heat, they open secondaries at about 3500rpm. You can relocate these to under battery tray quite easily, again worth doing if your tracking it.

Also, think about maybe taking the UIM off and giving it a decoke/clean, they can get quite dirty depending on use/mileage. Also while UIM is off just check the wiring loom that runs through there as some peeps had problems with that, again due to the heat.

As said water impellors can be an issue, but think they were mainly early black plastic impellors, my 2000 ST24 had a cream coloured impellor, so you maybe fine, but I changed mine for metal one as they are cheap.

Good luck with your new car, only real problem with the ST200 was the colour!!! lol, so I went for ST24 instead, but both great cars.