I'm a bit concerned that the OP on the M3Cutters website is an 'expert' in pinking, but then goes on to say that it's a good opportunity to get your valve clearances checked whilst you've got the head off.
Surely he should be doing regular valve clearance checks anyway, but I bet he didn't even check them when trying to diagnose this rattle.
He's also gone and replaced more than just the head gasket, so he can't state for certain the the head gasket caused this noise, and could have easily been the tensioner/guides that he replaced.
These engine are tappety, and you and you can hear it more on a light throttle as the engine noise is not drowning it out.
However, on a 5 owner, 100k mile car, I'd not be surprised for it to be a bit more rattly, as some owners don't bother paying the extra £200-£300 to get the valve clearances done.
Valve clearances are the first thing I get done whenever I buy an ///M, as I can't guarantee how the previous owner has maintained the car - or how well/completely a dealer has carried out the work they've been paid for.
Your best option is to compare it to a few similar cars and see if there's much difference - as you don't want to have to fork out £1000 if the dealer investigates and finds nothing wrong.