3. Aperture
The choice of aperture is linked to the choice of lens and the format, but essentially the answer is the same. Purchase a lens with the largest aperture you can afford. The ability to reduce depth of field and blur the background is crucial to many portraits as is the requirement to sometimes work in very low available light. With a large aperture lens you can always close the aperture down if you require more depth of field but with a smaller aperture lens you do not have the ability to reduce depth of field to artistic levels. On my 85mm lens (for the 35mm full frame format) the maximum aperture is f1.2, on my medium format camera 150mm lens the aperture is f3.2, but lenses for larger format cameras appear have naturally shallower depth of field due to the larger format size and effective camera to subject distance. For my 35mm camera my f1.2, 85mm lens is an exceptional but expensive piece of glass. If you are restricted on budget then at the very minimum aim for an 85mm or 100mm lens with a maximum aperture of 1.8 for portraiture.