50mm: I think 50/1.4 is the most usable of Fifties. Great for portraiture wide-open, sharp stopped-down just a little, can be used on tubes for macro, lighter than a f/1.2. Other Fifties are great too but if I could only have one [shudder] it would be the FA or SuperTak 50/1.4.
Macro tubes: A-type tubes are handy but costly. A good kludge is to get a couple old A-type TCs and remove the glass. Two deglassed TCs will push a 50mm lens to 1:1 magnification. That's for AF or A-type lenses; for M42s, just use cheap tubes.
Macro lenses: 100mm is more generally usable outdoors than are shorter lenses. You don't get so close to bugs that you scare them all away. If you're not shooting live bugs, or otherwise don't need a lot of elbow room for working, then a shorter lens is fine, and the 50mm+tubes combo is economical.
For a lot more on this, see:
https://www.pentaxforums.com/forums/pentax-lens-articles/152336-cheap-macro-b...lose-work.html. Cheers!