i would keep it simple and limited, when it comes to theory. The Communist Manifesto, Principles of Communist, Socialism: Utopian and Scientific, Civil War in France, Critique of the Gotha Programme, Capital Vol. 1 - that's more than enough. if you like, you can replace Capital with books like Lenin's Imperialism or State and Revolution.

something i would keep in mind - at the end of the day Marx was true to his roots as a philosopher. i would be surprised if you can pick up an 'introductory' or 'easy' text by Marx without taking notes and reading a second or third time.

when reading, questions you may ask: "Marx argues [point], but why? Is this argument convincing? There are a lot of words I'm familiar with here, is he using them in the common sense, or is he referencing something else? Is that his own theory and what he personally believes, or is he making a point about another thinker (i.e., Proudhon, Hegel, Smith, Ricardo, Lassalle, Feuerbach)? What assumptions is he making? Why should I believe this? Why do I agree/disagree with this?"

If you agree with everything you read on first encounter, without questioning or doubting it, you might've misread something or you might be missing context. I, for example, had to read Critique of the Gotha Programme multiple times because on first time I was careless and failed to distinguish between Lassalle and Marx, and the second time I failed to distinguish between 'wealth' and 'value' - both mistakes are so wrong I would've been better reading nothing.