i wanna know which version of apple watch os it would be in if i purchase it today

Once you get the Apple Watch and you have paired it to your iPhone, you follow the software update instructions, and you will be at watchOS 5.2.1

There’s no way to know for sure. It depends on when the watch shipped and in stock watches are not updated before they are handed out to customers. Why do you want to now? The current version of watchOS is 5.2.1