Assuming that one should only learn secular topics for the purpose of parnassah (making a living), to what extent may one learn those topics? Is it only permitted to learn enough to get a job that requires the least amount of knowledge? May one learn more in order to get a better job so he may live more comfortably and/or perform hiddur mitzvah? What if one must sacrifice a certain amount of Torah learning now in order to do more secular learning, but having a better job will facilitate much more time to learn Torah later?
Rabbi Shnuer Zalman of Liadi (author of Shulchan Aruch Harav) writes in Tanya Chapter 8 (English translation/commentary from here) that it is permitted to learn secular topics in order to secure a livelihood:
וכן העוסק בחכמות אומות העולם בכלל דברים בטלים יחשב לענין עון ביטול תורה, כמו שכתוב בהלכות תלמוד תורה...אלא אם כן עושה אותן קרדום לחתוך בהן, דהיינו כדי להתפרנס מהן בריוח לעבוד ה׳ או שיודע להשתמש בהן לעבודת ה׳ או לתורתו, וזהו טעמו של הרמב״ם ורמב״ן ז״ל וסיעתן שעסקו בהן.
Occupying oneself with the sciences of the nations of the world is likewise included in the category of engaging in inconsequential matters insofar as the sin of neglecting the Torah is concerned (for in studying the sciences of the nations, too, one is guilty of neglecting Torah study), as is explained in the Laws of Torah Study (Shulchan Aruch Harav Hilchos Talmud Torah 3:7) . . unless one employs them (these sciences) as a useful instrument, viz., as a means of earning a more affluent livelihood with which to be able to serve G‑d, or unless he knows how to apply them (the sciences) in the service of G‑d or to his better understanding of His Torah; (e.g., he utilizes mathematics to better understand the laws of the Sanctification of the New Moon.) This is the reason why Maimonides and Nachmanides, of blessed memory, and their peers, engaged in them.
In the Chassidus Mevueres explanation on Tanya they explain (footnote 50) based on the comments of the Lubavitcher Rebbe that by this the Alter Rebbe intends that even if one could survive minimally without a secular education - if by studying such topics he will have a more comfortable livelihood which he can utilize to serve Hashem on a higher level, this is permitted. The gauge would be that if such additional income is simply bettering his physical standards it would not be allowed, whereas if it will uplift his spiritual service it is.
Rabbi Yishmael says in Menachot (99b) that one cannot learn "Greek Wisdom" because it would take away time from learning Torah and the passuk says " לא ימוש ספר התורה הזה מפיך". However, the Gemara in Berachot 35b discusses this passuk, and while R. Shimon b. Yochai says one should not work, R. Yishmael says it is something one should do as it says "ואספת דגנך". This raises an apparent contradiction in R. Yishamel's position on what לא ימוש means.
Some explain R. Yishmael's view by saying that he only allows one to take off time from learning for Parnasah, but not for learning "Greek Wisdom". Some then equate Greek Wisdom with secular studies, which would create a prohibition to study secular studies if it isn't for earning a living. (However, it does not seem like we hold like R. Yishmael in Menachot, and Greek Wisdom is probably much narrower than all secular studies, see this post.)
The Gemara in Berachot continues:
אמר אביי: הרבה עשו כרבי ישמעאל - ועלתה בידן, כרבי שמעון בן יוחי - ולא עלתה בידן. אמר להו רבא לרבנן: במטותא מינייכו, ביומי ניסן וביומי תשרי לא תתחזו קמאי, כי היכי דלא תטרדו במזונייכו כולא שתא.
Abaye says that R. Yishmael's approach worked out better in practice. Rava is then quoted as telling his students not to come before him [to learn] during Nissan and Tishrei so that they won't need to spend as much time on parnasah during the year.
Rava's statement shows that one should sacrifice time from learning at one point in a year if it will leave him with more time to learn later. This argument can probably be extended to learning less torah for a couple of years if it really will leave a person with more time to learn later.
What about choosing a career based on personal preference that will take more time to study for but without providing more time later? I think this would also be permitted even according to those who only allow studying for a livelihood. Since the studies are for a livelihood, they would be included in the "heter". One is not required to make calculations about which would take the least amount of time away from learning without regarding any other factors. (Even by more severe prohibitions, if something is permitted, one often does not need to make calculations to find the least violations.)
See also the Mishnah at the end of Kiddushin which discusses what livelihood one should teach his son. While it does mention teaching " אומנות נקיה וקלה", it doesn't seem that a career should be avoided just because it involves more training time. There are many other important factors. In fact the whole question may be more complex than something that can just be looked up in the "shulchan aruch", since it involves lifetime personal decisions.