I also suggest going with plain light DME instead of dark. The reason I say this is because eventually if you go all-grain, you may want to convert this recipe. It's very hard to convert dark malt extract into an all-grain form. I'd look around at different recipes on the forums here and just wing it with the grains while trying to keep it somewhat similair to what others have done.
Personally, I like the grains you've got so far. The only thing I might do is add a 1/2lb or so of black patent and then maybe up the Caramel 60 closer to a lb. That's just me though. Substitute the Dark extract for plain light and you'll have a decent porter.