The answer to your question is no. 1.5x is the crop factor of the camera so you are correct at multiplying the lens focal length by 1.5. The lens is still the lens though. The aperture is an iris similar to the pupil in your eye that opens and closes allowing more or less light into the camera thus to the camera sensor. So basically the aperture does not change.
As for buying old lenses. I have no experience with using an M42 lens on my dslr however keep an eye out for M and FA series lenses as they are quite cheap. Some FA lenses are still in production and are just as expensive as DA series but some of the FA series are a bit cheap used. Do not confuse the Pentax FA with Pentax FAJ which are the worst series of lenses ever produced by Pentax. The FA series also have auto focus which I consider a valued bonus in some situations. Happy lens hunting