Sorry to hear of your troubles, it's something we seem to be seeing more and more of unfortunately yet sadly there's no cheap way to fix it. What I find more worrying is that there's no real set pattern or confirmed cause for the failures which to my mind, means it could just as easily happen again further down the line.....or not as the case may be!
When I was at CPC last year I mentioned this to them (they had 2 in for worn cams at the time!) and they seemed to think they saw an unusually high number of z4ms with this issue relative to the numbers produced. Their take on it was perhaps they are driven harder or are tracked more often as it's something more common to the csl. I wonder though if it's perhaps the opposite and it's due to the fact that many of these cars are kept for weekends (like many CSL's I imagine) and used infrequently meaning the oil retention issue mentioned in the link by Babw comes in to play.
As for the HG, I'm not aware of an uprated one but there may be an improved version out there. That said, given that failures aren't hugely common (and very, very rarely on cars with less than 50k) I think a standard OEM one should be more than fine. Same for the clutch imo, though I'd be happy to spend a little more on this rather than chancing it with a cheapy. No idea where's best to shop though I'm afraid.
