I would have a look at videos on youtube to get some inspiration. Lisa Eldridge is great, Sharon Farrell and Pixiwoo are also good. There are loads of people doing makeup videos.
Personally, I don't think one brand does everything well, so I wouldn't want to buy everything from one brand. I also have an issue with the hygiene aspect of having the testers applied on my face/lips/eyes. I am not convinced that the sales people are as good at cleaning the brushes they use and so on.
If you buy new products, make sure you try them before the day so you know that they will last and look good all day.
Regarding primer, powder and setting spray, I think it depends on how well your makeup lasts. If you know it already lasts really well all day, then you might not need to add more products, but primer and setting spray might be helpful if you find that your makeup doesn't last or if you would like it to last longer. Powder is great for touchups.
There are loads of advice online regarding flashback and foundations. I would test any makeup by taking flash photos before the day to make sure it looks right.
I really like Laura Mercier for tinted moisturisers, cream eyeshadows and lipsticks, Estee Lauder for Double Wear mascara and Double Wear Light foundations, Mac for powder blushers, pressed powders, matte lipsticks and brushes, Guerlain for lipsticks and gloss and Meteorites powders, Maybelline for gel eyeliner.