First off, don't buy a dye - buy a rinse. The grey will end up lighter than the rest of your darker hair giving you highlights. I loved that stage. Once you're mostly grey you pretty much have to go with dye which I found pretty much did away with the highlights. If you're out in the sun a lot, the dye will lighten up the exposed strands.

I can't tell you about quantity because I'm a chicken and go to the hairdressers.

C