car runs again!
You guys were right. We took it to Benz Friendz in Seattle and they said "All this car needs is to have the heads rebuilt and new chain & related components installed."
Turns out that only 2 valves hit pistons (chain broke on startup).
So Benz Friendz did the left head (the right head was apparantly fine) and the chain, tensioner, et al. Doing this also cured the 380SE's infernal "tickticktick" of lifter noise at idle. It now idles quietly, like an MB V-8 should.
He also had the right fron window regulator done, since that window hadn't opened for a while.
Total cost? $2297 US, with parts and labor.
With the cash we were able to save, my dad also picked up a '91 Volvo 940SE with 207K miles, but with many new components and a new engine at 115K miles (previous owner ran car out of oil). This car is turbocharged and has Volvo's rare Turbo Plus boost control system, which provides a temporary overboost at full throttle above 3,700 rpms, raising HP from 162 without the Turbo Plus to 188 with the system engaged. Lots of passing power.
The important thing is that the MB has a new lease on life and is smoother and quieter than ever. It was a fun drive from Seattle to Wenatchee, via I-90.