I'm sorry to hear Exchange didn't work out for you. But thanks for giving us a try and I'm glad to hear the tutorials were helpful.
Deactivating and deleting Exchange should be all you need to do. If you want to remove everything from the database as well, you can do so. In your wp-config.php file, you'll see a wp_debug option. If this is set to true, at the bottom of the General Settings page in Exchange (Exchange > Settings), you'll see the "Dangerous Settings" option. Check the Reset Exchange box then save your settings. This will remove everything Exchange from your database, including any PayPal API keys that you entered.
When setting up PayPal Standard Secure, you did enter some URLs into PayPal (Instant Payment Notifications (IPN) and PayPal Auto Return) to connect to your site. You can remove those from PayPal, too.
This should take care of everything for you, but let us know if you have any additional questions.