Definitely figure out what was going on with your ankle. A little pain after 300k could very well translate into something much bigger and requiring significant healing time after 600k. It may have something to do with your cleats or shoes, too. FWIW, on my most recent brevet (a 600k) I had some pain right around the ankle bone on the inside of my right ankle. I've noticed recently that all my right SPD pedals have gotten fairly loose, and my right cleats generally wear faster than my left because that's the foot I usually step down with. These two combined to make my right foot feel much less stable than usual, and I suspect that was the cause of the pain. It wasn't bad and went away within a few days, but it was strange because it's never happened before. New pedals and cleats are definitely on the shopping list.
As far as sleep goes on the 600k, keep an open mind. All the American 600's I've done had a sleep stop at around 350-400k in a motel, and at least one drop bag, and I'd sleep for 1.5-3 hrs or so, plus change clothes. But I've been in Germany for the last two years, and I've done three 600's here, and none of them had bag drops. One of them had a tent with cots which I didn't use, and the others had no sleeping provisions at all. So I carried a little more in terms of extra warm clothing, but didn't change shorts and didn't take a long sleep break. But in a way, this does give you a little more freedom because it means that if you do take a sleep stop, you can do it on your own schedule when you need it instead of whenever it comes up. Personally, I get sleepiest right around the time when it's getting light, regardless of whether I've already had my sleep stop at that point or not. A 20-min. nap early in the morning makes all the difference, even if it's just leaning against a corner in a gas station or bakery or something. If you want to sleep longer than that, the rooms with ATM's in them are great because they're generally warm and dry and you can bring your bike in with you. A lot of the riders in Germany carry those reflective space blanket type emergency sleeping bangs, which weigh practically nothing and don't breathe especially well but will keep you warm enough to sleep for a couple hours.
The biggest difference between a 300k and a 600k is riding through the night. You have the time beforehand, so if you've never done it before and you're concerned, you might try going out for an all night ride before that, starting in the evening and finishing in the morning. It'll be a good opportunity to test out your lighting and see what it feels like, even though you won't have ridden all day, and you'll have a little better an idea of what to expect from your mind as well as your body.