Trying to keep track of the different models is painful to someone who hasn't been researching the type for 20 years. Is there anywhere a nice little comparison chart that shows most of the stuff a Mooney shopper would be interested in, so they can narrow down their search to a few variants?
Let me see... The main differences are the engines and body length. But first, some of the "quirky" models:
* The original M20 (150hp) and M20A (180hp) had a wood wing and tail. There's an AD that eventually required the wood tails to be replaced with metal. Not a whole lot of these birds out there any more.
* The M20D was originally supposed to be a trainer, and had fixed gear. A lot of these have had retracts added.
* The M20L "PFM" had a single-lever-controlled Porsche engine. Most of these have been replaced with a more traditional airplane engine.
Body length: Up to the M20E was the "short body" version. M20F through M20K/252 are the "medium" body, and the M20L and later are the long body.
Engines:
M20B, M20C, M20D, and M20G have a 180hp Lycoming O-360.
M20E, M20F, M20J have a 200hp Lycoming IO-360.
M20K: 210-220hp Continental TSIO-360
M20M "Bravo": 270hp Lycoming TIO-540
M20R "Ovation": 280hp Continental IO-550 (Newer Ovation3 models use an STC'd 310hp version of the same engine.)
M20S "Eagle": 244hp Continental IO-550
M20TN "Acclaim": 280hp Continental TSIO-550
The most popular models: M20C is 180hp short body, M20F is 200hp mid body, M20J is an aerodynamically cleaner M20F, M20K is a turbo'd M20J. M20R Ovation is 280hp long body, M20M Bravo is a 270hp+turbo long body, M20S Eagle is a somewhat-pointless lower-powered version of the Ovation, and the M20TN Acclaim is an Ovation with turbo that's been cleaned up and will do over 235 KTAS at altitude.
Hope this helps.