Here are some sources that I feel are valuable reading for this topic:
Rabbi Willig:
Torah U Madda – Theory and Practice (Audio)
Secular Studies: Are they for everyone (Torah U Madda Journal Volume 1: 1989)
Rabbi Yitzchok Breitowitz:
Full-time Learning vs. Combining Learning with a Profession
Part II – Entering the Professional World: Pros and Cons
[Eyes to See: Financing Torah Study with Public Funds is only Permissible for Distinguished Scholars Completing Their Halachic Training, Pg 452, (Rabbi Yom Yov Schwarz)]
"Tur (OC 156) states that after davening in shul each morning and spending some time in the daily study of Torah, one should "then turn to his business activites, for ‘all Torah study that is not accompanied by work will cease in the the end. Commenting on this, the Bais Yosef writes:
In other words, do not say, "Why do you tell him to turn to his business activities? It would be preferable for him to continue studying Torah forever! Therefore, [the Tur] explains that it is a desirable practice for him to turn to his business activities, as we learn in the Mishna (Avot 2:2), "All Torah study that is not accompanied by work will cease in the end" That is, he will run out of money and be forced to search for a livelihood, and then he will not even be able to set aside limited periods of Torah study each day. It will also lead him to sin, for poverty will cause him to violate the will of his creator"
KIDDUSHIN: CHAPTER 4: MISHNAH 14 (Kehati)
Rabbi Yehudah says, A bachelor may not herd cattle, and two bachelors may not sleep in one cloak; but the Sages permit. Anyone who deals with women may not be alone with the women. And a man may not teach his son a craft among women. Rabbi Meir says, A man should always teach his son a clean and easy craft and he should pray to the One to Whom riches and possessions belong; for there is no craft in which there is not poverty and wealth, for poverty does not come from the craft, and wealth does not come from the craft, but everything is according to his merit. Rabbi Shimon ben Eliezer says, Have you ever seen a beast or bird engage in a craft? And they sustain themselves without trouble, and were they not created only to serve me? And I was created to serve my Creator – does it not follow that I should sustain myself without trouble? But I have behaved badly in my deeds, and I have forfeited my sustenance.Abba Gurion of Sidon says in the name of Abba Guria, A person should not teach his son to be an ass-driver, a camel-driver, a hairdresser, a sailor, a shepherd, or a shopkeeper, For their craft is the craft of robbers. Rabbi Yehudah says in his name, Most of the ass-drivers are evil men, and most of the camel-drivers are straightforward; most of the sailors are pious; the best of the physicians to gehinnom, and the most straightforward of the butchers is the partner of Amalek. Rabbi Nehorai says, I put aside all the crafts in the world and I teach my son only Torah, for a person eats of its reward in this world, and the principal remains for the World to Come. But all the other crafts are not so. When a person comes to illness, or to old age, or to troubles, and he cannot engage in his craft, then he dies of hunger, but Torah is not so, for it protects him from all evil in his youth and provides him with a future and hope in his old age. Regarding his youth, what does it say? "But they that wait for the Lord shall renew their strength" (Isa. 40:31). Regarding his old age, what does it say? "They shall still bring forth fruit in their old age" (Ps. 92:15). And similarly it says regarding Abraham our father, peace be unto him, "And Abraham was old…and the Lord had blessed Abraham in all things" (Gen. 24:1). We find that Abraham fulfilled the entire Torah before it was given, as it is written, "Because that Abraham hearkened to My voice, and kept My charge, My commandments, My statutes, and My laws" (Gen. 26:5).
[See http://www.moreshet.net/oldsite/mishna/5761/19-08-01/friday.htm for the Kehati on this mishna]