I have wondered one thing in Mr Collins history.
His father was "miserly" and one can suppose it was at least partly an origin to his fightings with Mr Bennet.
Still his father could pay him university studies.
So when I read that he had a "very sufficient income", I understand it as coming not only from Hunsford rectorate, but also from the inheritance of his father's, which had also allowed him the improvements he made to the parsonage (as in the novel, it appears clearly that this improvements were made by him, though he seeked Lady Catherine's approbation). He was preparing a nest for his future wife... ;-)