By 2005 most of the bugs were sorted and the 4.2 litre engine has many improvements over the original 4.0litre which, in itself, in AJ26 or AJ27 form is a very sweet engine provided that the items that Grahame mentioned are dealt with.
In the UK, corrosion is the big problem. So, inspect floor pans, areas around the suspension mountings front and rear, sills, wheel arches etc. Speaking of suspension, the front seems to be prone to the upper and lower fulcrum bushes seizing on their respective bolts. Check for any creaks or stiffness in the suspension.
Likewise, operate the convertible top. It should complete and open or a close cycle in about 20 to 25 seconds with the engine running. Also, look for any drips of hydraulic fluid around the left hand side of the header close to the hydraulic latch.
Transmissions are allegedly sealed for life but all the advice is to get the fluid and filter changed every 50k miles.
I bought my XK8 as a project but I absolutely love driving it. I also have a 4.0litre S Type which I wouldn’t change for anything more modern. Both came from the same designer but you can observe difference in the way they are built, coming from different factories. The XK8 shows its XJS heritage although it shares its electronics architecture with XJ6 and I think you in particular, will find that side of the vehicle very interesting.
I’m sure you will enjoy owning an XK8 and if you can find a rust free one its value is likely to rise rather than fall.