I'd agree with mirod's XML::LibXSLT recommendation. If the server is linux based, the underlying libraries can be installed relatively easily via RPMs.

Sablotron is also very good and has RPM's available. It requires expat but that's probably already installed.

Of the modules you suggested, XML::XSLT would undoubtedly be the smallest headache for your ISP but also unfortunately the biggest headache for you :-(. Its XSLT implementation is incomplete and not actively maintained, so save yourself the pain.