We have had an '98 e36 M3 for nine years and still love it!
I have heard plenty of issues with some of the older(1990s-2000s) V8 and cooling problems, some autos don't hold up, it's not a Toyota, they NEED maintenance.
We have replaced the radiator, water pump, expansion tank(x3) valve cover gasket and now the suspension is needing a little TLC, however it has over 131K on it.
If I had the means I would get a newer one, but they are just too expensive, new BMWs don't have a dipstick so you can check your oil
sorry if it's not of much help.