The exhaust system is pretty expensive. A new rear muffler assembly will likely cost you $600. A center resonator probably around $400. Aftermarket is not much cheaper, and is considerably poorer quality. I put an aftermarket muffler on mine, only because it has a lifetime warranty. I'm sure an OE muffler will last three times as long, but hey... I won't have to pay to replace it as long as I own the car, ad the things come off and on in 20 minutes. Have a somebody check on the timing chain , and especially the rails and guides for the timing chain. Chains on this car should be replaced about every 100K and you'll be getting close to that in this car. My 1986 420SEL has 170,000 miles and the most expensive thing I've done to date was that muffler last week. You're brakes are probably getting thin if they have been replaced in the last 30,000 miles. Its not cheap to put rotors and new discs, but its not bad either, and the OE is definitly worth it. As for the air bag, they were supposed to be changed at a given date, but I think that was extended, I'm not sure how long...check with a dealer. These are excellent cars, and if you don't skimp on maintanence and parts they only get better with age. Tires are important too. Mine had Pirellis P600s on them when I bought it. Excellent for handling, but the ride was harsh, also they wear like sidwalk chalk. I use Michellin M+S 330s in the winter and Continental ContactTouring (OE size is P205 65VR15). Both handle reallu well, are comfortable (low noise and better ride). You're buying a heavy car so buy tires that can handle it.
Good Luck to you, if you have any more questions please email me.