Gio: UNDERTAKE has strong implications. In the simplest connotation, it just means "to begin" something. But the stronger connotation is that it means to assume a powerful responsibility. Debt of course, is a money issue. So if one "undertakes" debt, it has serious implications. It puts one at risk. For example, if I adopt a child, I UNDERTAKE a serious responsibility for that life. In business, any project a company UNDERTAKES can involve millions and even billions of dollars. So UNDERTAKE is any beginning with strong implications. There are exceptions of course. I can UNDERTAKE the task of answering a student's question here on Italki. I can UNDERTAKE the making of a pot of coffee. Generally though, the word UNDERTAKE implies that some serious matter of personal responsibility is the issue.

This would more commonly be written as "how much debt to take on." So the phrase means, "figure out what you are going to use the money for and what return you will get on it before you make the commitment to borrow and repay it."

undertake means 1. To deliberately begin to do (something): undertake a task. 2. To pledge or commit oneself (to do something): undertake to care for an elderly relative. 3. To promise or guarantee: "The man undertook that one room at least should be ready for occupation the next day" or in Italian intraprendere! ;)

I don't know much about finance but I understand there are financial markets for buying and selling debt. In this financial context, to "undertake" a debt is to accept the liability to pay a debt.