Ondine was my first Ravel piece. If you love it (as I did, and I do... ) and you feel you have technique to play it, go ahead. It's difficult but not exaggeratedly difficult. It's very pianistic, in my opinion easier than Scarbo, Alborada or Toccata. I don't think you need a Ravel "preparatory" piece in order to play it, just my opinion and only if you have technique and background enough. It would be interesting to know your background, it is, which difficult pieces have you play until now.