You do not need to purchase a domain. That's entirely up to you. You can create a successful blog without purchasing one at all. If you do want to purchase one go here > Dashboard > Store > My Upgrades after your read what's here:
WordPress.com uses PayPal as the payment processor, and as such, can only accept payment from countries they support: https://www.paypal.com/worldwide/
In addition to major credit cards Mastercard, VISA, American Express, Discover, and PayPal https://www.paypal.com/worldwide/ WordPress.com can accept echecks and some debit cards.
The debit card must have a VISA or Mastercard symbol on it, and should work just like a credit card when selecting the VISA or Mastercard option on the payment form, even though it's a debit card.
More about echecks here: