It works really well. I used an earlier version - but you know what - you learn to live without HTML quite quickly!
Scripts in posts? That's always a big no no. Does IPB allow this? Are you literally talking about posts accepting <script> code? Potentially a massive security hole there if it's exploited.
Not sure what script you're referring to with regards to PayPal. Donate buttons can be made with simple image and URLs:
Anything more advanced is possible using things like BB Code and Media Sites.
No IPB, does not either, they allow HTML though; although for the paypal buy now buttons, we created something using the BB code, and a bit of modifying.
The scripts, I was really thinking about like Aweber email capture scripts, etc. as well as paypal.
Posts can only contain BB Code. The add-on I mentioned above (http://xenforo.com/community/resources/html-bbcode-permission-protected.731/) can allow you to parse HTML code within special BB Code tags and assign permissions to its use but it isn't a good idea to give your users the ability to render HTML. That's what BB Code is for.
You will need some knowledge/research of regular expressions though, and you need to be very careful not to incorrectly replace things or miss things.
I'd say if it's only 200, it's probably not going to take that long to do it by hand by adding the Parsehtml tags if you can get help to get it to work on XenPorta. Obviously with 10,000 posts like I had, that wouldn't have been possible by hand.