Can I use it for a newborn or do I need to buy another part?
In the complete lie-flat, it theoretically can be used with a newborn. However, I personally wouldn't do that, the newborn baby is very open and actually needs a more enclosed, safe space to feel protected. Thus, you can either use it like that with a newborn only very occasionally (e.g., for the trips to the doctor), or get a universal cocoon or thinner footmuff or an insert soft cot to create that space for the baby. That could work, still, it needs to be said that this is a travel, occasional-use stroller, and the suspension and wheels are not really the best to regularly use with a fragile newborn baby - something a bit more substantial is much more suitable.
Your -very berry- Eli.