I deal with HID kits almost every week and have a lot of experience installing them.
I second the guys, 6000K is turning blueish slightly, 5000K is pure white. When it comes to 35W vs 55W, you need to get 55W kit, they are obviously brighter, but you may have problems with 35W kits. OEM bulbs are 55W and it is recommended to stay within the same wattage. Some 35W will not work with E60 ECU, it will not throw an error, but it will be flashing. Not very noticeably, but it may. It depends on the quality of the kits.
Please do not get the cheapest kit. You should be looking to spend at least 60 EUR. Some kits are more expensive than the others for a reason. Also look at the wiring, get a kit that has couple small boxes integrated in ballast wiring (they are identical to "warning cancelers") and make sure they are CANBUS, something like this:
Are you going to do the installation yourself? You'll need to make a 1 EUR coin size hole in your headlight cover caps for ballast wiring.
Are you familiar with NCT HID Xenon regulations? This is what they check for.
National Car Test Manual 2012 - page 30 - HEADLAMP CONDITION:
"Where HID (High Intensity Discharge) bulbs are fitted switch on the headlights and check that cleaning devices such as headlamp
wipers or high pressure water jets are present and work. Check where possible that the headlamp levelling device is working."