You’ve already said you’re planning on it, but Cagliostro is a must. It’s not only a good Lupin film but a classic piece of anime media by Miyazaki.

I’m nostalgic for Season 2 (red jacket) lupin, and while you probably not need to watch them all I think there are quite a few fun and good episodes there too (including a couple of cool and beautiful episodes directed by Miyazaki, which have some interesting relations to his other work).

Also I think the new Lupin the IIIrd film series by Takeshi Koike is quite cool, representing a much darker and grittier Lupin than the light-hearted Miyazaki vision. It’s a great time to get into these ones too, because the final 2 pieces, Zenigata and the Two Lupins and The Immortal Bloodline are coming out in just a few weeks.