In Unified Process methodology, the Supplemental Specification contains all the non-functional requirements. (The functional requirements are specified with use-cases.)
Some additional non-functional requirements are currently covered in the Vision Document.
Reasonably modern system:
Slower systems should work, but XML/XSLT processing is CPU-intensive, so you'll be much happier with a fast system.
Since we are initially focusing on Java, any system that can run Java 1.4 or later should be capable of using UP-lite. We'll be looking for volunteers to port, document, and support using UP-lite on various Operating Systems.