My first pregnancy, I looked homeless. I wore regular clothes as long as I could and was uncomfortable, bought maternity clothes near the end of my pregnancy.
With Son, when I started to show I purchased key, cheap pieces. Wash and wore them to the ground.
For sure purchase maternity pants/jeans, bra and undies. You can try and swing the rest but maternity clothes are made for a reason and life is so much more enjoyable comfy when pregos. It doesn't hurt to purchase one of each thing and see how it goes, eventually you will have to buy - I recommend doing it early.
Buy one nice maternity outfit/dress for your baby shower and events. Make sure it's nice/pricier, wear and repeat. It's important to feel good on special days when with child.