Mr C
At £4k you're not gonna get a bike with super low miles, but the rocker problem is related to poor oil feed when the engine is first started so miles could be irrelevant, its more about warm up procedure. Worse case scenario is it needs some rockers and at £100 each that can be expensive, but worse case I've seen (at Cornerspeed) was like £1000 worth, Nelly managed to aquire some SPS heads complete with perfect rockers and fitted them for about £600, so its not that bad and a performance improvement. My SPS needed 4 rockers at about 6k miles even with good warm up procedure, so just budget for it. If you buy from a dealer get a warranty and get Nelly at Cornerspeed to check the rockers while its still under warranty.
The regulator will fail (£100 odd) the neutral light switch will pack up (£20) the oil pressure switch will go (£20 off I think) the clutch will need replacing at some point (£100 odd) and other stuff will vibrate loose but its all part of the charm ! Go for it - the riding experience is worth it !!!