Well, I will chime in now if I may. I have a 1985 380SE. I do not think I agree with the comment "worst engine in 30 years" that was made. I have 268K on mine and it is still running good, aside from a vacuum leak somewhere causing a stumble when cold and now my PS box seal that blew out/hose the other morning. I have NO oil leaks and NO oil burning. I now have 3300 miles on my oil and it has not leaked down even a quarter of an inch. Pretty amazing for a 268K mile car. Yes, it has lifter ticking and the engine does not sound quiet as a mouse, but still running nevertheless.
Powerful? No, they are not. This is a SMALL V8 and this is a heavy sedan. Also, the 84-85 have the double row chain, but it seems like it is either the tensioner, guides or rails that fail. I will admit that my car has 268K and I have NO idea when they were last changed. I will let the future owner decide that. The car will be back up for sale after I get the PS repaired.. probably around $1K or so, give or take a hundred or two?
Luckily, my car has (for now anyway) bypassed some of the very common problems on these. Everything works save for the cruise control, power antenna, auxillary fan and the LR window will not roll down, as it seems to be off of the track and of course now, my PS is out, due to a busted seal/hose that decided to burst Monday morning @ 9:30am EST, gushing PS fluid all over my driveway.
No rust on the undercarriage (but it is a southern car), but it has some starting to bubble up under the paint above the RR fenderwell, but this car was in DC (or maybe it was New York ?) for the first 3 years, which may explain that. Luckily, no rear window leaks either (so far), but my old antenna seal was leaking, allowing some water in the RR floor, but that seal was replaced and problem was solved. Oh, MPG? Someone said 14-17 in town? I get around 20 mpg in town, even with the stumbling/rough cold idle. I guess that is why I hate to sell it, is the good mileage I get with it. If it were properly tuned, I would say it may get 23-25 mpg?
BTW- there were no "late 80s" 380SE (not that I know of anyway). It was 1984-85. 1986, the 420SEL took over, on a longer wheelbase. As mentioned, there was a 380SEL (LWB), from 1981-83 I think. However, I think these have the single row t-chain.
Good luck.