Does the wife's purchase of land and the husband's building upon it constitute disobedience to a displeased mother?

If the money belongs to your wife, then you are not obligated to obey your mother in preventing your wife from buying the land, because obedience to the mother is obligatory in what is good and not in preventing the wife from disposing of her own money.

However, if the money is yours, then you are obliged to obey your mother in not buying the land, unless you would be harmed by it. If you would be harmed, then there is no objection to buying it. You should try to appease your mother.