Wow a lot of questions. Remember the color chart for kelvins in 35watt? When moving up to 50/55 watt setups if you choose 6000k the light will actually be 1 level down so it would look like a 5000k. I don't recommend 50watt kits, just not worth it IMHO. I have run just about every kelvin temperature from 4300k to 10,000k. In my opinion 6000k is the best all around.
If you have a kit that flickers while driving its often due to poor quality Chinese kits, with weak ballasts, an often times a pooe grounding source. A lot of people will add a simple $1.25 capacitor from radioshack to fix the problem. I have done it, but dont like to di that. Don't think all Chinese kits are junk. They aren't. There are 2 companies that I use for my customers needs. 1 is DDM Tuning, the other is 3B Specialties, with the preference of the later of the 2. My reason is it is a lot easier for me to talk to them and tell them the vehicle I'm working with, and what they need and I get a complete all inclusive kit with all relays, capacitors, and or error eliminators built into it, while maintaining OEM harness looks at a great price ( no I don't pay their website prices), and get it shipped overnight to me.