Which X3 to buy?

[Doru]

I have to get rid of my Tribby pretty soon and replace it with a little X3 (for my daughter), and yes, I will be taking care of it. She'll do the driving.

I am interested in the 3l engine, so that would be a start, however I am unsure which year would be the best.
I understand the pre-facelift models have the M54 engine (with which I am very comfortable), and the facelift sport the N52 engine. Also reading through different BMW forums, it reveals that the new 6 speed slush box had some issues (not sure how effective updating the software is - I can also do that, I have INPA, GT1, SSS Progman, etc and I am familiar with procedures).

To conclude, what are the common issues with pre-facelift & facelift models?
Do the pre-facelift models have cooling issues like the e39 for example (exploding radiators, hoses, expansion tanks)? The like.
Is the N52B30 engine more reliable than the M54B30? I know the M54B30 is quite a good one, didn't give me any headaches so far (except for the known CCV issue). Does the N52 suffer the same?

Thanks, looking forward for some pointers.