I am going to look at a 74 300 D. Only known problem is it won't turn off with the key. What could this be? How hard is it to fix? Any other things to look for?

Thanks

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87 300SDL 220K
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82 diesel Chevette 440K
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83 4.1L buick powered cutlass THE TOY

I'll take a stab at this one as one of mine has this condition.
Shut-off is by vacuum. If the door locks (vacuum operated also)work well and will open and close a few times after the engine is shut off then it may be the "shut off" located on the back end of the ignition pump. If that's the only known problem you've got a real good one as it should be fairly cheap to diagnose and fix. I'm sure there have to be more problems than that.
Good luck with it.