LL - you can express breastmilk as soon as your baby is born.
At first it will be colostrum - which is especially rich in antibodoes & growth factors & then when it will be mature milk when your breastmilk comes in around days 3-5. If a baby is born prem then a mum can express her milk straight away - same for mums of term babies.
Colostrum is produced in a very small amounts but this is fine. If you are able to breastfeed your baby then it can be easier to express your milk once supply & demand have been worked out between your baby & you and you are both confident with the new skill - even if you've bf before every little baby is different! For the breastmilk supply to become established can take around 3-5 weeks - so after this is will be easier - but if you need to express beforehand it is ok as well.
On the pump - it just depends on what your needs are. If you will only express occassionally then a hand pump is cheaper & doesn't require batteries - but is more tiring on the hand at first. The electrical pumps are heavier & make a low noise but they can extract more milk quicker - so if you're returning to work & expressing then this is an advantage.