I’d be interested to see what codecs are available for transcode. That version of Final Cut was released in 2004, and I’d forgotten that ProRes was introduced in 2007 (!!)
You could try Uncompressed (massive files, but lossless) as a mezzanine on your way to ProRes on the new machine.
With FCP 4.5 XML exports (or any version through FC 7), you’ll need to run it through a translator first. I think Premiere will read them natively, but I use an app called SendToX. The old .fcp project files themselves cannot be opened by any version of FCPX, nothing to do with platform/processor.