Hi there new mummy to be!
I'm sure your anxieties will vanish when you see you new baby. I agree that raising kids in this day and age is a costly affair, but in my opinion the second and third child isn't as big a financial adjustment.
Some tips I can give you is to re-use baby clothes you used for your elder kids and even request baby clothes and other goods like a stroller, baby bath, carrier etc. from parent who are giving them away. This will significantly lessen the financial burden once your baby is born. You can also plan ahead by not giving away your older kids clothes and toys and keeping them in storage for when you baby needs them.
As for your question about avoiding getting pregnant again, this is a question you need to ask your gynae at your next visit. There are several precautions you can take from simple ones as taking the contraceptive pill and using a condom.
Good luck with everything!