You can get HID conversion kits easily off eBay, which you may well know already. The bulb type for the IS200 is HB4/9006. Yes, the original headlight units are fine with HIDs in, it's a dead easy conversion (the HID bulb just plugs straight into place, with the HID ballast being connected to the existing headlamp wiring).
Colour is a personal preference, but I think that 6000K is quite common around here, a good compromise IMHO between brightness and looking cool :D . 4500K would be a bit brighter but a bit whiter too. 8000K would be less bright and more blue.