If u aim for 120, u'll be bored grinding ur crew in SS class alone. U'll need other ships, EBB/PBB/AA to help grinding easier, so having more than 1 BO, 2 main gunners and 2 AA gunners would help.

Most ppl have different setup/preference on their 'ideal' crew. 

Normal 120-140 vet on +12 sailor crew (torp, sonar, planesman, engys) with on time classing would do well on KM SS.

And KM SS last tier rather odd since it only use 1 torpman and no gunner at all on R slot.

If you only like to play ss6 

Then 1 BO, 1 sonar, 1 planeman, 1 torp. and 5 engines.  

If you like to play other ss,

you need one more torp and gunners.

As mentioned above, you may train your crews with ecl, pca, and pbb/ebb. Then you need gunners, scout, and maybe aa (I dont like aa)

In term of training,

I like use engines (11 or 12 repair/ 11 or 12 engine). Definitely, the hero, 90 lvl neutral, or legend sailor is the best, while they are not tradable. For engine, just class on time and dont over vet them.
I am using a bo 15pot/12rep and like it. 
Planeman elite pot sailor and class on time.
My sonar is 15pot/12rep and late class, as said by many ppl that ability for sonar is capped.

torps(11 torp/12rep). I classed them on time. You may consult with more ppl. because torp ability is capped soon. 

For the engineers, is it necessary to class them to chief engineers on time or just stick with 1st engineer