Don't think anyone on here has an XR8 of either variety (sierra or falcon) as these cars were only available in s.africa (sierra) or australia (falcon). I dare say there is a few have been shipped abroad but never seen any mention on here.
Best thing to do is what you obviously are doing already - google, google, then google some more
That said, I would imagine that the same advice applies to lookinjg at one as most other cars - check for oil leaks, overheating, crud in the exspansion tank, worn down pedal rubbers and seats on suposed low milers, check wheels for alignment, panel gaps for regularity and all the stuff you probs already know - hope it turns out well for ya...and welcome to the boards here - you could be the resident xr8 expert