Also, looking around, I see that prices for Space Empires IV have really dropped. I'm very tempted to get it now. Though, I see the reason is because Space Empires V is what people are after. Maybe I should skip straight to V?
I've been playing since Space Empires II in the early-mid 90's. Space Empires IV is awesome, especially with some of the mods out there. The AI is pretty poor overall but if you like sandbox-style gameplay and robust ship design flexibility, it's hard to beat. Combat does tend to devolve into a combination of either warp point assaults or planetary assaults, and not much else. But hey. . . . the game let's you build your own ringworlds and Dyson Spheres!
I had high hopes for the modding flexibility of SE5 but the UI and sub-par 3D engine (especially for ground combat) utterly killed it for me. To this day I still play SE4. Good 2D icons actually work better than a hideous 3D engine (even if you import sexy, high-poly ship models like I did for Star Wars mod, the engine itself is lacking. . . . it's hard to describe).
I have 3 mods for SE4: someone's Star Trek: TNG-era mod, a Babylon 5 mod with a bunch of my personal changes, and my own Star Wars mod (where I play both the Clone Wars-era Republic and all the major Separatist factions, in an accurately-mapped Star Wars galaxy).
I highly recommend the Proportions Mod for SE4, probably the most flexible research tree and ship components. Nothing is as flexible as Aurora but Proportions Mod might be close.