There is nothing as 75watts HID. If shopkeeper is honest, he will tell you not to buy it. Original 35watts HID that comes in Honda Spark is much more brighter than local 55watts china made. Only benefit you gonna get with 75watts is instant lighting up. No waiting time. My friend is using 55watts in his 2009 Vitz and his chrome has turned purple around the bulb. HID is not supposed to be installed in reflector lights but in projectors. If you want the pics of purple chrome, i can send you right away. In Vitz there are 2 options to install hids. One with H4 bulb having halogen on low beam and hid on high beam. Problem with this is you gonna get very poor light on low beam and no focus. On high beam it gives out good light but its not "high" beam actually. It wont hit the back view mirror of the car in your front.
Other option is to get H4-3 bulb. It has a magnet in it which moves the hid bulb producing a perfect low and high beam. Low is low with excellent focus and high hits the front cars badly. Problem with this bulb is it will stop working anytime. Magnetic motor is not durable and the dedicated harness gets water damaged.
HIDs suits best in projectors or the lights having separate bulb for low and high(Corolla, Civic etc).
As far as chrome damage is concerned, i wouldnt recommend 75watts at all. Even the original 55watts can turn the chrome purple.
Get some good quality 55watts hids having 6000k bulb in it. It will give you the best possible light. Check the bulbs before you buy them as they say its 6000k but they actually are 8000k with a blue tint. Choose the bulbs with pure white light having no traces of blue tint.