I'm tentatively coming out of my dark ages, but before I go farther I'd like some suggestions on what to buy.

I recently bought the 75218 X-wing and the 75212 Falcon, and I had a great time putting them together, but just following instructions was never why I liked legos as a kid. I see all of these amazing MOCs, and I want to get into building my own stuff, but because my old legos got parceled out as gifts ages ago I only have the pieces from those two sets. I don't feel like I have anywhere near enough legos to build much that I would find interesting.

Do you guys have any recommendations on sets that are worth buying that will help me build a generally useful core collection of parts? More to the point I really want to make nice looking space ships and chunky mecha. I'm not super concerned about minifigures, although I wouldn't mind getting some classic Space and M-tron guys somewhere down the line. I figure I'd need to go to the after-market for those.