Has the SDM failures been dropping off lately?

Yes but... there is a different 'reported' failure rate between first and newer batches making the DA* 16-50 the biggest victim.

Most Pentax primes don't have SDM motors. IIRC only the 55, the 200 and the 300 have SDM motors. All the others are definitely 'safe harbour'.

All motors will eventually fail if not serviced regurlarly. It doesn't matter if they are Pentax, Tamron, Sigma, Canon, Nikon or Rolls-Royce.

Yes on the sensor and the money, no on the body. The K-5 may have similar build to D300 but is a lot smaller in size. It's size is between D7000 and D5100.

camerasize.com/compare/#187,188

You're wellcome.

Have fun,
Luis