Diaper bag, towels, wipes, little things like that... not to mention its a fun get together for friends n family :) 7-8 months is good. I had mine at around 7 1/2 months :)
And you can have a baby shower around 7 or 8 months so then you have enough time to get everything else you don't get in the shower
Your also gonna need shampoo, soap, and a lil grooming set... I'm getting a few 0-3 and then bigger sizes since they grow fast in the beginning
I would get 0to3. Even if the baby is small 0to3 wont be that much bigger. When you go to your dr. Appt you should ask the dr. how big and long the baby is, so you can get an idea.