The XR6 didnt come out until the EB Series II, so if you see a Series I XR6, Dont but it!
As for Problems, check for oil leaks around the front of the Head and the Rear Main. As its a Falcon there will be oil leaks, but obviously if theres too many then forget about it.
If you looking for an Auto, then make sure it has been serviced regulary.
Watch out for faulty Central locking, but it isnt really anything major.
The manual is slight faster from what ive seen, but only when you drive it at 11tenths ;
They already have some mild extractors on them so I would probably just chuck on a 2.5inch Cat-back system.
There are a few XR6 owners on here that should be able to help more though.