While yer doin' those fuel filters, you might think about replacing the primer pump. Especially if you see evidence of fuel leakage.
Check the throttle bushing on the firewall to make sure it's not degraded.
Check the air cleaner housing for play. May need new rubber air cleaner mounts (3) and/or new mounting bracket. U-pipe should be secure and not leaking air. Make sure oil drain in air cleaner is properly inserted in drain pipe.
Check for leaks on diesel overflow/recirculation hose. Hoses at fuel tank too.

Some of these are obvious - others not so.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)