Short answers, pulled from the story.
The first volume of Das Kapital appeared in February 1867 while Karl Marx was still alive. This publication occurred just days after Marx urged Friedrich Engels to read Honoré de Balzac's The Unknown Masterpiece.
Friedrich Engels completed the editing of Volumes II and III from Karl Marx's notes and published them in 1885 and 1894 respectively. Engels provided crucial monetary support that allowed Marx to continue writing despite suffering from liver problems and carbuncles.
Karl Marx drew heavily from Georg Wilhelm Friedrich Hegel, Ludwig Feuerbach, Baruch Spinoza, Gottfried Wilhelm Leibniz, Immanuel Kant, and Aristotle. He also engaged with French Physiocrats like François Quesnay and Anne Robert Jacques Turgot alongside Jean Charles Léonard de Sismondi.
Karl Marx defines value as an immaterial social substance representing congealed abstract human labour under normal production conditions. The magnitude of this value depends on socially necessary labour time required to produce goods with average skill levels.
Karl Marx faced dire financial situations for decades which forced him to rely on the generosity of his friend Friedrich Engels. His health issues including liver problems and carbuncles were directly caused by the stress of his work and lack of funds.