Peadar... the engine is actually quite reliable if it gets the chain guides and tensioner replaced with the new redesigned parts.
My R56 developed the rattle at around 28k kms.
Garage replaced guides under warranty but used the same bad 1st generation parts... so 3k km later... same issue.
BMW/MINI didn't want to do another replacement for free... so fought them on it... and they backed down and replaced them and tensioner for free again - but this time with the newly upgraded parts. Car has done more than a 100k since that and never another issue.
I would not be too worried about any rattly chain issues as most have long been fixed by now and can always ask whether it was done. Common thing to go on them is window regulators as they get a lot of use with the windows dropping down a bit when you open the doors. Easy to check that though and not too dear to fix.
They are much better on fuel... I got avg of 37Mpg, or 34mpg if driving her hard, and on long journeys closer to 44mpg.
Never managed to get more than about 30mpg out of the R53 and about 27mpg if pushing them hard.
More power too than the R53, and hardly any lag.
The R56 is not half as raw though, much quieter, more comfort etc, but that might suit you with a family.