Can I buy after I have landed?

Yes, if you have internet to install with, such as airport Wi-Fi. Buying and installing before you travel is far easier, since it does not depend on Wi-Fi of unknown quality.

Installation needs an internet connection, which is exactly what you lack on landing, so you must find Wi-Fi first.

If you must buy after arriving, use airport or accommodation Wi-Fi and complete the installation before you leave that spot.