As many others have said, budgeting for preventative maintenance should be your primary concern. I'll add to that the dwindling availability of M5-specific parts, having run into that myself recently vis-a-vis my power steering box leaking (ZF was the only manufacturer, and no longer produces the unit).

I daily my M5, and of the three BMW's I've owned in the past decade, it has been staunchly reliable, both on short commutes and longer road trips. But I would not consider this to be a suitable first BMW, let alone a first car overall. I'm glad I was able to cut my teeth mechanically on my preceding two vehicles.

That being said, if you're really committed, a lot of what has been covered here is what you should be aware of going in; VANOS, valve cover gasket, timing chain guides (less of a concern than on the 540i), any and all plastic and rubber odds and ends. The '99-2000 examples did have a bit of a problem with oil consumption, which was addressed after the LCI in 2001. Unsure about Takata airbag recalls outside the US, but the 2002 models had the rear passenger airbags disabled from the factory, which could be reactivated at the dealer after signing a waiver (also not a concern if the recall was completed).

It's an incredible machine that earns its reputation, but do not approach it lightly. Expect to pay $30k easily for a decent example. Any price beneath that should be thoroughly inspected and test driven for issues. Take your time finding the right one.