[hirofkd]

For the first question, the answer is it depends. If the hood looks pretty much like a stock with a couple of vents added or something like that, then start from the kit part. If the desired hood looks nothing like stock, do some scratchbuilding, using styrene sheet and putty. But I think it's usually easier to start from the kit part.

For the second question, I recommend that you use white primer as the base coat, add TS-16 yellow and then coat it with TS-65 pearl clear to simulate the mica look. If you want to sand, polish and wax to create a mirror-like surface, add clear coat on top of TS-65. Make sure to build protective layers by applying several mist coats on each color.