You should definitely start it, and work on it, and then decide if you are ready to bring it to the stage, and to really tackle the numerous problems. I say with these things, the sooner the better. Personally since it is such a large work, I would spend a lot of time at the beginning figuring out where the music is going, how to make all the transitions.
Then I start cataloguing things, ie, all the octave passages, all the "fast fingers" passages, all the transition passages, all the main theme passages, cantabile passages, polyphonic passages, super-soft passages, et cetera. I would practice from the first page forward, and from the last page backward.
The idea is, even if you can't solve every technical problem now, you will get so familiar with the piece you know it inside out. Let's say you have trouble with octaves in general (hypothetical). You study the piece, you know the whole structure, you know how to make the transitions, but you just can't get the ocatves right.
Two years later, you've played Funerailles, you've played Chopin octave etude, you've played Scriabin op.8 no.12, and octaves are much easier. You can go to the Liszt sonata, have all the knowledge that you had before, so all you have to add is a bit of practice on those passages. If you really want to learn the piece, but know for sure certain things you can't master now, don't let that stop you from getting to know it as best you can.
Walter Ramsey