Dealer level diagnostic equipment is not needed for certain models (roughly 6-7 years old). By then there are usually software leaks to the public. Right now BMW started a new generation with F chassis cars, but its still not impossible to diagnose with current programs out now.
I'm using Bavarian Technic professional diagnostic equipment, I also have INPA, DIS, SSS, NCS.
New versions are popping up on forums sometimes. The Bavarian Technic is more than what I need for the E60.
I always tell people if they are troubleshooting their BMW without diagnostics then they in for a treat.
There are guys running tunes on their N54's without data logs, then when they have issues they use cheap readers to try and find the problem.
Like I said if you do your research and gather the info you should know what to avoid, and what to expect.
I'm no qualified tech, but the equipment that I purchase (expensive) has been more than enough to keep my vehicles out of the dealership all these years. I still by my oil and filters at the dealer so they see some type of purchase by me,
but my cars haven't been there in a while.
If I do run into an impossible problem that even an independent mechanic can't fix then a dealer visit is in order, and they would probably be just as stumped (heard of that before).
Like I said its really not difficult to maintain your vehicle from your home with the right tools, that's why they are available to the public. BMW wouldn't sell their oil over the counter if they didn't want you to change it yourself.
The only thing that prohibits any work at home is time, laziness (rare) or injuries. If all those check out,
you're good. Have a CEL? Diagnose, hit the forum search button, buy parts, remove, code, install. That's the norm.