They always shake a bit at idle. The later engines (~83+) had a rack damper with a much stiffer damper spring. I took the one off my old 85 injection pump and put on my 84 car, but didn't notice a difference (fully screwed in). The two types are the same length. The new design has a gold coating.

The worst part of the shaking is the air cleaner. It tends to hit a vibrational "natural resonance" at certain idle speeds, which helps eventually crack the metal bracket. Those brackets used to cost ~$100, but reproductions now ~$16. The rubber mounts between the air cleaner and bracket also break, but real cheap. 1985 CA cars have a different frame mounted air cleaner. I may try to fit mine to the 83 engine now in my 85. But the filter costs $45. Another approach might be adding dampers to the manifold mounted bracket.

A quick way to judge the engine condition is to remove the PCV plastic tube and note how much ring blow-by comes out the hole. Leave the oil fill cap in. The used 83 engine I just installed looks new inside (peering through the oil cap) and shows no blow-by at all, so maybe a recent rebuild and I lucked out (from PickNPull via craigslist). My 84 engine shows noticeable puffing out the little PCV hole (less noticeable from the larger oil fill cap). That is probably more typical. My son took it back to college so I can't test more. To test further, get the $35 (sale) diesel compression test kit from Harbor Freight. However, must remove injectors, which isn't trivial. If so, plan on having the injectors pop tested, maybe 2 wk downtime.