Some engines are really touchy with even just one glow plug bad. I had a diesel once that absolutly refused to come to life cold with just one open glow plug. Yet otherwise seemed to be in good condition enginewise. It made little sense at the time but it was what it was.

Adjusting valves or at least checking them for decent clearances is very important. Many of these cars have gone tremendous amounts of milage since the last time done. The manufacturere recommends every fifteen thousand miles.