I have a lot of experience with this piece. Like you, many years ago I decided it was my ultimate ambition. In my experience it is every bit as difficult as it sounds! Having said that, that only makes success taste all the better in the long run!
I made my first attempt whilst at music college, and performed it at a performance workshop. Whilst it was exciting to play, it is fair to say that for much of the piece I didn't manage to control it in performance.
From time to time after this, I got it out and tried to work on it, deciding that it was getting a bit better each time, then around 18 months ago (6 years after the first performance) I decided I was finally ready to take it on in public again, this time in a professional recital. It was the last piece before the interval. It still had a few rough moments, but by in large I felt I pulled it off and it felt amazing to finally do it.
So, in answer to your questions. Firstly, whether to buy it yet. This depends entirely on your philosophy. I think no harm in buying it to know your enemy, then keep revisiting it if it feels too much of a stretch at the moment.
In practising it, patience is the ultimate key. This will require more slow practise than just about any other piece. I remember having lessons with Ian Clarke on this piece, who said something like 'When learning music, you do all the careful learning of the notes, until the point where you know them and can rely on them. That is except with Chant de Linos, where no matter how much you practise, the notes always seem to be just out of reach' Not trying to put you off, just an idea of the long road to learning it! Also, Chant de Linos is an exciting and adrenaline filled piece. As such, I always practised feeling as calm unexcited as possible when playing to counteract this, so when the adrenaline kicks in during performance, you have a chance of maintaining control. Furthermore, I always tried to concentrate on the beauty of the tone, as it is all to easy to end up playing aggressively with little quality. Rhythm is the most vital aspect to the piece, especially in relating to the piano. Personally, I found it very useful to work out some cheat fingerings on a few of the nastiest runs. If you get stuck into learning it, drop me a message and I can share some with you.
Also, if you're interested in Jolivet, do you know the Incantations for Solo Flute? They're worth a look. Also I'm doing the Sonatine for Flute and Clarinet at the moment which is great fun. Not as tough as Chant de Linos!
Funnily enough Chant de Linos is back in my life now as I'm accompanying it for the first time soon. Learning the piano part has also been quite a project, but very satisfying to get to know the piece from that side. Now I understand the bits where I gave my pianist a hard time!