1. you need to open http on the computercraft config to allow a connect. Opens your server to hacking.
2. i don't know how computercraft will handle high end internet code(java,php,..etc)
3. You'd be capturing people paypal passwords, without encryption. They may not like this, and i'm sure paypal wouldn't like this.
Entirely possible. Http api querying a page containing commands, which you input in a page that is password protected. You would even be able to send out data using get/post. The only "issue" is that it requires a web server of some sorts, but you could use one of the many free web hosting sites out there.
To the original OP: display a QR code that a person can scan with their smartphones to open a donation link?