The brake warning light probably just means your brake pads have worn down to the wear indicator. Quite easy to check depth of pads visually and changing pads is a fairly straight forward job. I think the warning triangle is just to attract your attention to a fault or warning.
New pads should sort it but worth checking the condition of the disks as they are fairly inexpensive parts.
If your planning on keeping the car then a Bentley Publishers workshop manual is a great investment.
https://www.amazon.co.uk/BMW-Z3-Serv.../dp/0837616174