You could go to your local PPG or DuPont paint store and get a can of vinyl paint in each color and use a spray gun or have them put it in a spray can for you. If you dont know where to do try going to a Body Shop and see if they can get the paint for you or find out where they get there paint...
would probably be easier to find a whole roof. Most of the time when they install a sunroof they cut the roof bows that run across the roof under the sheet metal. Just welding a piece of sheet metal in the opening and not having the roof bows in would cause it to rather floppy.
Depending on the paint you have it should work. I would use something like SEM vinyl dye (PPG and DuPont both make it too) it will wear off over time though.
The header panel is the fiberglass panel in the front of the car with the headlights and grill in. I dont really know about the aftermarket panels for the buick but if they are any thing like the ones for other cars they should fit fairly good and and you wont be able to tell the difference...
It is actually not that bad just as long as you remember to keep the direction of the pad going off an edge at all times and keep the speed of the polisher on the slower side. once you start to polish the car you will just seem to work the best at a certin speed. It is just going to take...
If you are trying to polish the car after sanding I personally prefer to use a polisher that is not DA since it usually cuts faster you have to be a little more careful along edges though. I have a Milwaukee Milwaukee Electric Tool - 5460-6, 7/9 in. Dial Speed Control Polisher, Polishers...
The emblem adhesive works good but it is kind of messy. 3M makes some nameplate repair tape that is thinner then the normal double sided tape. You can cut it out with an Exato knife. Time consuming but looks good when you are done. Here is a link to the nameplate tape Tapes for Automotive Trim...