As already done in a comment, I hereby suggest the free Visual Studio Express editions, which are often overlooked when looking for free tools. All Express editions feature XML and XSLT Intellisense and red squiggles, and support for using a schema for Intellisense in XML. I don't know how far things like XSLT debugging go in the Express edition. I suggest the Web Developer edition, but you can install all of them on one system anyway (unlike the Borland Turbo Explorer editions). If you later need more, you can upgrade to a commercial edition without having to relearn everything. Even if you later switch to Liquid XML Studio, it's still Visual Studio.
XSL:FO is not explicitly supported, I think, but if you have a schema, you have at least Intellisense and squiggles for it.
