A couple of suggestions:
1. Which armies are 'good' changes relatively frequently. Also, while 40k is pretty poorly balanced, you can absolutely win lots of games with any codex, particularly if you don't mind using allies. I can't think of a single codex that I couldn't build an army out of that I'd be happy to take to a tournament (at least if all I wanted was winning).
2. Because of that, I'd chose an army based on what you like the look and fluff of. Those also change, but nearly so quickly.
3. If you're going Marines, and it looks like you are, unless you really like the look/fluff of a particular chapter (and it doesn't seem like you do), I'd come up with a paint scheme of your own and use the army for whichever chapter you like on a roving basis. That way you don't get locked in, particularly at this point when you don't know what you want to play.
4. Don't ask anyone, particularly a bunch of people you don't know on a forum, what a good list for X army is. You'll get way too many opinions to make sense of, and most of them will be wrong. Try to borrow some models and get a few games in to see what you like. Pick a few units that you know you'll want to use because you basically have to (i.e. assault squads for BA, Tactical squads for SM, and so forth). Basically, instead of trying to figure out what a good 500 point army is, try to figure out what a basic 500 point army is that will be good for learning the game, and use that.