Stephanie wrote:

"(Colonel Brandon) has not inherited yet, and somehow has the money to pay her debts, put her into comfortable lodgings, with nursing care, and, afterwards, to support little Eliza."

I don't think CB would have found it too difficult to meet the expense of rescuing Mrs Brandon and supporting young Eliza at school.

If he did not have sufficient money to cover Mrs Brandon's debts then he would have had to borrow, but would have paid off the debt as soon as possible. We know from his stewardship of Delaford that he is financially prudent. He does not spend what money he has on a lavish lifestyle - wine and women, hunters and curricles.

I'm sure he provided Mrs b with the best care he could afford, but it would not have been for very long, as this passage shows:
Life could do nothing for her, beyond giving time for a better preparation for death; and that was given. I saw her placed in comfortable lodgings, and under proper attendants; I visited her every day during the rest of her short life; I was with her in her last moments.

The Colonel then supported young Eliza at school. I imagine this would have been at a respectable but modest establishment such as Mrs Goddard's school in 'Emma', rather than an overpriced fashionable academy for young ladies like the one attended by Charlotte Jennings.

Bear in mind that although the Colonel was not then a wealthy man, he did not have the to meet the expenses of maintaining a home, and a wife and children.

He probably inherited some money of his own after the death of his mother, and again when his father died, even though Delaford and the lion's share of the family's money went to his elder brother.

He also had his Army pay. The link below from the trusty L&T archives gives some information about relative incomes of military men and others at various levels of society.
Another L&T archive post ('Army Pay Table' November 17, 1998), gives the daily rates of pay for army officers - my arithmetic suggests that a Captain would be paid about £113 a year, and a Major would get around £170.
These are modest sums, but the Colonel was a modest man. Most men at that time had to manage on smaller incomes than that, yet still managed to support a wife and children.