You need to be psychologically ready to take the plunge. Do not buy then have buyer's remorse, it is frowned on and a huge pity on what should be a happy and special occasion. I would just take my time to do research and feel comfortable and knowledgeable with the specs, like leather, colour and size before buying a birkin. Or maybe, it is just not the right bag for you, or not the right 1st H bag.
From my experience, hesitation is not a good thing, it means your gut instinct is trying to tell you something. I would just stop before a huge mistake is made. If you love the bag you should be feeling happy and excited, not stressed.
FYI, you could return a birkin, but you will only get store credit back. And the store credit expires in one year. Not to mention your cred with this store would go down the drain too. So do make sure you are happy with your purchase and avoid going down this road.