Having owned several older Volvo models with the "factory A/C" units, I would strongly suggest you stay away from these, unless you feel the need to keep the car "original". The late 60's to early 70's underdash units were only marginal when new, even though the standards were much lower then. No way will one of these old units really keep a 140 cool on a sunny 100 degree day. They would barely do the job in an 1800, with much less space to cool, and a bunch less glass. Every one of these I ever owned was prone to evaporator freeze-up... they just had to work too hard.

Someone here suggested Vintage Air, and that is probably a good place to start. In the long run, you'll probably spend very little more money there than you would by overhauling something you find in the salvage yard. And with the VA (or similar) system, you'll more than likely actually keep the car cool. Incidentally, if you're using the stock radiator fan on your 142, you will need something better. The VA system probably has an electric fan, at least as an option... you'll need it, especially if you do any city driving in hot weather.
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1971 142E ITB racer, 1973 1800ES, 2002 S60 T5