How easy is it to change spark plugs, oil, oil filter, air filter, trans fluid, and the fuel filter on a 97-98 540i or a 97-up M3? I'm interested in buying one of the above, but don't want a car I cannot maintain myself.

To be honest, if you've changed them on one car, you've changed them for all. Only thing is locations may differ. I don't think it would be that much different than another car. Unless you start getting into the whole computer thing, then you would need some experience.

Take a loock at http://www.bmwtips.com/
and you will find pictures and text telling you how to do it. It's a lot of info on that page, changing spark plug is under tips&trics.