I just bought a 2001 330ci and when i did research i asked some BMW mech. that i know and a guy i know that works with turner motorsports and they all told me the same thing the 540 and the 740 are harder to work on and more on parts. the 3 series are easy to work on and the parts are cheaper unless it is an M3. I dont know about the 525 but i would stick to the 3 series if i were you. I love my new car im sure you will be happy with what every BMW you get?
hope this helps
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