Diapers, definitely. They will be using it a lot especially during the first few months.

It won't matter much during the first 1-2 months. Can just get NB or S size diapers. However, different diapers got different cuttings, not all sizes are the same across brands.
Otherwise, if you are still worried, you can just get M size because eventually the baby will grow bigger - then can use

You can get the more expensive ones such as applecrumby, mamypoko and merries. They are expensive but very very comfortable. I am sure she would appreciate it!