Lots of experience will let you guage the difficulties of a particular piece better than anything else. I would suggest reading through Ondine a few times at whatever tempo you need to get a good understanding of the technique involved.
Often, a piece is much more daunting at a glance than it is once you start working on it, and just as often what seems easy becomes terribly difficult at a more rapid tempo.
If I don't believe I'm ready for a piece, I usually don't pick repertoire geared toward it; I simply learn anything else that I might happen to love, content in the fact that when I'm ready I'll know and not before then. However, if there are pieces you love that happen to be close to Ondine, by all means take that route. I would suggest Chopin's Op. 25 No. 6 etude (I'd like to say that simply working on some of Chopin's etudes helps tremendously, whether or not you "finish" the etude), Ravel's Barque sur l'Ocean from Miroirs, Ravel's Jeux D'eau, and Bach fugues.
One never really knows that one piece will definitely lead to another - as such, I take the opposite approach and learn as varied a repertoire as possible without thinking about logical progressions from one piece to another. A wider technical perspective will give you both the ability and the confidence to approach literally any piece without feeling like it's beyond your abilities. It took me three or four years of very serious practice before I really felt "ready for anything," and I think there's no substitute for learning a very large volume of repertoire.
Ondine might seem like a ridiculous proposition at the moment, but you'll feel ready eventually, and when you do... good luck!