OK, so I've decided to attack a planet on my list of scanned planets. The current owner is a much lower rank (I almost feel bad), and I should be able to win.

Questions:
1 - Can I "colonize" the planet? Or am I still restricted because it hasn't been 4 ranks since my last colonization? Or do they just lose everything on the planet but retain ownership?
2 - How much energy is typically involved in a planetary invasion - how many separate attacks at 5 energies each should I save up for?
3 - Anything about the process that is not straightforward. I'm thinking attack a number of times, then win, then possibly take over (colonize) for myself?

1: You need to have an empty palnet slot to invade a planet. So no, not yet.
2: Depends on the planet, if it has no defense, [(Planet HP)-10]/20, if it has some defense, id reccomend getting ready for about 5 extra attacks. It costs 15 energy to invade as well.
3: Theres always a chance at failing to invade a planet, to increase this chance stack Attack, though watch out the chance caps at 90% so theres always a chance of failing.

also take notice, it is DOUBLE the cost to invade a planet then it is to colonize it. definitely learned my lesson early on that one

Money is superfluous if you got enough good mining planets