is there any difference between part numbers? Is there any difference between iPad Air part numbers? Like ll/a versus zp/e – for different countries.

I want buy a new iPad Air and I stock between different part numbers.
The difference is localization. For iPad Air, it only means that you will get translated documentation. So it's OK to get it from another country like the Philippines.

Warranty is worldwide. The software language you will choose on first use, and you can change it later.

The only real difference is for products with keyboards (iMac, MacBook, etc), obviously then it will influence your keyboard layout (QWERTY, AZERTY etc.).