The API can be used with all Xero plans, but not all features will necessarily be available. Payroll API requires a Payroll plan. Cashbook and Ledger plans exclude certain features (e.g. invoicing) but can still be connected to via the API.
The Xero API uses very few of the defaults that can be set through the Xero UI. The only defaults it will use are the tax rate from the account code if the tax rate isn't sent, and the description, account code and price on inventory items (but not tax rate). All other information must be specified in your call.
It's best to become familiar with the Xero platform and basic accounting principles before designing an integration. Xero accounts are free, and each comes with a fully functional Demo Company. The Demo company is populated with sample data to give you an idea of what items should look like. We also have an extensive Help Centre with information on each feature as well as how-to guides specific to the API. You may also want to consult with a Xero Certified Adviser who can instruct you on the accounting requirements many clients may have.