do I need to decide now if I want the watch to have a cellular account?
do I need to decide now if the watch has a cellular plan or can it be added later?
Question marked as
If you buy a GPS + Cellular Apple Watch, you do not need to activate the cellular right away. You can do it at a later time.
If you buy a GPS Only Apple Watch, it does not have a cellular radio, so it can never use cellular. However as long as your iPhone is nearby, the GPS only Apple Watch can do everything the cellular model can do, by using the iPhone’s radios over the Bluetooth connection.