For wordpress plugins you can see the below links
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http://wpmu.org/8-wordpress-paypal-plugins-for-receiving-payments-and-donations/
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http://www.tipsandtricks-hq.com/wordpress-simple-paypal-shopping-cart-plugin-768
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http://www.bestwpplugins.net/wordpress-plugins-for-paypal-integration.html
Check this plugin link it might work in your case and its tested in wordpress 3.1 its working fine.
url: http://wordpress.org/extend/plugins/wordpress-easy-paypal-payment-or-donation-accept-plugin/
To customize the paypal payment and thankyou message you can code as per your requirement but u need to know the paypal api how it interact.
To know about paypal html variable see the link:
To know more about paypal payment form see the good tutorial on net tuts+
url: http://net.tutsplus.com/tutorials/html-css-techniques/creating-a-paypal-payment-form/