The setup doesn't provide much info. Can I change to a different site after I sign up, or would I have to open another Adwords account, or can I use one account for multiple sites? If I open an Adwords account, do I have to buy ads right away, or could I research and put off paying for my adords till later?
You can advertise multiple sites with an adwords account. When you sign up your not obliged to start advertising straight away, you can start whenever you would like.
Right on. If you used a voucher to open the account, make sure you used the credit within the time limit, if mentioned.