The freeze plug should be an easy thing to source... pretty standard item. You just gotta get the right size. Forget about the block heater for now... you can replace it later, or get one that just goes inline on one of the hoses (though those don't work as well). You say its a '72 142s... is that to say its carburetted? Most of US spec '72 cars would have been injected, and they almost always start fine in cold weather.

After you get that freeze plug replaced, get a new thermostat. THats a cheap place to start, and usually cures the overheating problem. If it still overheats, then start looking into more expensive stuff such as a new/used radiator.

These are some of the most easy cars to work on, and learn on. Especially if it is carburetted. Keep it around, and keep up with maintainance, and it won't let you down. Almost nothing that can't be fixed roadside with minimal tools...

-Matt

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-Matt '70 145s, '65 1800s, '66 122s wagon, others inc. '53 XK120 FHC