I would pick a bm over a merc any day of the week, BM's are a lovely drive and e class mercs are more for old men, at least it still looks like you have a heartbeat if your driving a 5 series.

As for chipping a car with that miles, who knows really, you could chip a car with 300k and have no problems, where as you could chip a car with 30k and blow the turbo, but clearly there is a bigger risk the more miles it has on it.

you not fancy something like a 530d or a 540i?

I would stay clear of the 525d though if I were you, good luck