I had a P1800 ('69) for a few years and liked the looks and performance. Parts are reasonably available and the cars is easy to work on. However, it doesn't have all the confort and conveniences of a modern car. You have to question it's value as a daily driver.
The Flathood may be the most collectable of the 240's. (GT owners also have a right to argue for that title). It has documentable scarcity (about 470 sold to the public) as well as a nice "story". Most parts can be found on any contemporary 240, except maybe the grill. Performance is nice with the engine stock and can easily be improved. There are lots of upgades to improve the handling. I find it to be a nice place to install all the R-Sport stuff I have found over the years. No personal experience with the 120 series but I like to look at them at the shows. When I hit the Lottery, my first purchase will be a nice 123 GT :-)

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'93 850GLT
'83 242TI Flathood