N52 is as good as M54,but the water pump is $500+,fixing the ticking valve lifters may vary from $1k to $5k,transmission will be $3k+,self-destructing rear diff $2k+ and so on.
I meant for the OP,if his only reason to move to E70 is that he is afraid to spend a few thousands in the near future for the E53,it may backfire big time.If one wants to upgrade and is ready to spend more $$$ on the newer model that is perfectly fine.