As Garry said... The Pentax crop multiplier is 1.5 for all lenses. (because of the sensor size, same as nikon)

M42 adapters - get a 'good' one.

A 50mm f/1.7 can be had cheap off eBay etc.

Don't forget Sigma and Tamron lenses as options. The Sigma 24-70 or Tamron 28-75 (both f/2.8) are really good walk around options.
They also have 18-200ish options

Sigma (CR Kennedy) will price match grey imports.