Integrating Paypal donations – WordPress coding question

I’m a WordPress newbie, but a developer by trade, trying to help a non-profit with their website.

I need to integrate with paypal, to allow donations via Paypal; either one time or recuring donations. We also need to collect the customer’s information for tax receipt purposes.

Now for the questions.

Where do I put the code to do this? Is the best thing to do to write a plugin that contains the necessary php classes? If not, where do I put the php code to handle the transaction response from paypal?

If there is an existing plugin you know of that will save me writing this myself, please let me know.