Originally Posted by
Kuwagaton
1. If you're making your way through the Golden Age I'd imagine you have Action #1 for Lois, Superman #7 for Perry, and #13 for Jimmy. Lana and Pete haven't had their first appearances collected apparently. Wiki sites claim Steve's first appearance was reprinted, but it wasn't. The upcoming Byrne hardcover should have the first appearances of Hamilton, Cat, General Sam Lane, Maggie, and Turpin iirc.
2. Jimmy is easy. The Kirby collection, Strange Transformations trade, Nick Spencer's story, and the current series should be a satisfying cross section. Lois has her own trades too. Everyone else is really difficult because they're bit characters whose appearances across many years add up, instead of having considerably dramatic storylines. Between the Daily Planet tpb and Moore's Whatever Happened I think you come away with a good look at many of those characters without getting insanely specific (most of them aren't all that static anyway).
3. The first appearance of Supergirl has been reprinted a number of times but Man of Tomorrow archives also gives you John Corben, the original Metallo. His brother Roger takes over in Superman #310 but that hasn't been reprinted. You can find the golden age Metalo, Toyman, Prankster, a definitive old Luthor story, and some very early villains in the golden age omnibus vol 3. Mxy is in omnibus 4. Brainiac has his own trade, Silver Banshee will be in the Byrne hardcover.
4. That gets pretty subjective, easily moreso than the supporting cast but also easier to find in collection. Are you big on the versions from the cartoons, or current ongoings or anything? If I had an idea of what you like I think I can narrow down the recommendations.