Regarding XJs, don't buy a cheap one for £600
You can get a mint UK XJ8 4.0 sovereign that has had its cam chain tensioners done and has almost everything working for £1200.
I know this because I had one and sold it for £1200 a couple of years ago.
The only ones that have any resale value are the XJRs and the Daimler Supers and they really aren't worth the extra IMO.
I put 8,000 miles on mine, spent £1000 on fixing it/maintenance over about 15 months and I absolutely bliddy fecking loved it !!