I've had both an MX and K2 for over 30 years or so now and I think it they are both great film cameras. I can readily recommend either one to you with no reservations.
Personally, I like the viewfinder of the K2. In some ways I prefer it to the MX with the standard split screen with micro prisim collar. The standard focusing screen of the K2 was described a pentaprism with cross-microprism or split-image focusing screen, 0.88x magnification. It works quite well with just about any lens you may attach to the camera.
Unlike the MX, you can't swap out the standard focusing screen - at least you can't do it yourself. On the MX, with the standard screen, you can get half-moon black out areas with some lenses. This doesn't happen with the K2. I have all the screens for the MX and this is just one of the many great features of that camera. It solves the black out issue and it's nice to be able to buy extra toys/attachments for your camera. One feature the MX, LX and K2DMD have over the K2 is the Judas window. This feature allows you to see the F stop in the viewfinder and I find it really handy.
If you want to buy extra accessories for your camera, and who doesn't, you will want either an MX or LX. These two cameras offered more accessories than any other Pentax Manual focus cameras. This included stuff like: focusing screens, bulk film backs, data backs, winders, motor drives etc. The LX even had a very cool set of optional view finders. The K2DMD, as mentioned above had an optional data back and motor drive. The K2 had no options available.
As for the shutter of the K2 it does sound clunky at first. Once you get used to it it will seem less clunky than solid. You also get a faster flash sync with the K2 than you will get with either the MX or LX at 1/125 sec. I should also add setting the ASA on a K2 is a unique experience. Until you get used to it it will seem very strange. After a few rolls of film it will become less of an issue.
If you are buying a used film camera, be it an LX, K2 or MX you should still send it out for a CLA. Like a car these are precision instruments and need service from time to time to keep working at their best. The K2 will be over thirty years old now. Cameras of this era were designed to last a life time but they required some maintenance. You wouldn't drive your car for thirty years without changing the oil would you? You mentioned the K2 in question was last serviced in 1991. You should factor in the cost of a service to the price. This could add up to $150.00 to the bill.
You said you were looking for a tough camera with a good focusing screen. The K2 will fit the bill. The MX and LX are also great cameras and will offer more focusing screen options. As I recall the MX and LX shared the same focusing screens. There was a different screen for every focusing need. If focusing is a priority for you the viewfinder system of the LX is unmatched by any other camera. I don't think you can go far wrong with any of these cameras.
Last edited by 8540tomg; 08-09-2014 at 07:42 AM.
Reason: typo