Congratulations on your pregnancy!
Exclusively pumping from the start is very hard work and if possible, it’s easier to solely breast feed for the first 4-6 weeks to really get a good milk supply before looking at the complexities of expressing.
A breast pump will never be as effective as a baby when it comes to telling your body to produce milk, and nor will it be as effective as getting the milk out of your breast.
You can buy/rent hospital grade pumps but again, although they are fantastic, they aren’t as effective as building up a milk supply as a baby breastfeeding would.
Expressing is also very time consuming and it may be difficult to do it regularly when you have another child to care for, and unless you are expressing 8-10 times a day then your milk supply won’t really build.
Is it that you think expressing would work better for your circumstance than breast feeding would which is why you are considering it?
Have you spoken to your midwife about it? She could probably sign post you to some breast feeding specialists or breast feeding supporters who could talk to you in depth about the pros and cons of expressing.
There is also an infant feeding topic on Mumsnet where you will probably be able to read many women’s experiences of expressing in the early days and you may be better prepared for what to expect.