Read the reviews on this forum on M 42 cameras. The Spotmatic F enables full aperture metering with later Pentax Takumar Super Multi Coated lenses, and perhaps some third party lenses. Full aperture metering is nice, since IMHO the stop-down implementation of the Spotmatics is not as nice as that on the Praktica LTL, for example. Please, no hate mail! I am a loyal Pentaxian.
Other than that the Spotties are lovely cameras, beautifully made, smooth operating, elegant looking, and reliable. Since they seem to use a bridge circuit for metering a silver oxide cell will work fine in place of the original mercury cell, thank goodness! Of course with any camera that old a CLA should be factored into the cost.
After that you could move into the K mount series, with perhaps a KX. And perhaps later on the ultimate, the Mighty LX.