I'm going to sell my 120 this spring. It's a 2dr with o.d. Rust is always the problem with these cars, get the best body you can (mines structurally solid but definatly needs work, if you are interested in a decent restoration project it would be good). Something no one else mentioned is history. If these cars aren't used regularily the seals start failing and the rubber decays. A 120 that's been sitting for 1 yr or more will probably have brake problems as well. All of the above is repairable it just takes time...
One of the things I love about the 120 is how simple they are, oh yeah the windshield wipers gear assembly is weak and sloppy. I don't know were to get parts for it, i was lucky to find a good used one for mine. If you live in a rainy area, try to get one with wipers that aren't too sloppy (the slop shows up as wear on the passenger side windscreen trim). Oh yeah, the rear axle of the '66 and earlier model is badly designed (1/2 shafts prone to bending) the best thing to do get the later cars if you can (because I have a '66 I keep a spare axle in the basement). Other features on the later cars are collapsable steering column and dual brake lines.

Happy hunting,
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patrick of montreal, '66/120, '83/245.