Serpentine belts run around several pulleys, each with a bearing to carry the load. There isn't any particular bearing I know of that is the "usual offender". Isolating the noise is usually possible by listening with an automotive stethoscope (< $15 at most parts stores) or even a long screwdriver or combination wrench pressed to the component housing and just next to your ear.
You're listening for the noise that sounds out of place with the others. Most of the pulleys will make a smooth, whirring noise. A bearing on it's last legs will have a periodic scrape to it or just a continuous scratchy sound. Once you've identified the cause, you can estimate the repairs. Some are more expensive than others. I don't think a 1987 300E has the dreaded air pump just above the alternator (jump up and say "Thank you, Jesus!), so you're worst case is probably the water pump.