yes, trying them in your own hands is mandatory... M42 cameras will offer you a bunch of lenses at decent prices: Pentax, russians, german and rebranded lenses for all the tastes... so i'd suggest Practika, Pentax, Exa.
If you choose Pentax in K baionet (or Ricoh or other brands) you can always get a ring and use Ks and M42s as well.