Is it easy to see the players practrice ?
It depends where they practice. There are open practice courts that you can just walk up to and watch them practice on. There are also some that are covered, but if you stand at the top of court 4 you can view from up there. Last year caught almost everybody I was looking for practicing at some point or another.
Do you know a cheap hotel close to the stadium ?
The Comfort Inn in Flushing NY is a ten minute walk from the park. I stayed there last year, and it was just fine. Clean, nice staff, located in a safe area. I would recommend it.
If a hotel is too expensive, you should check ou the Flushing YMCA for a room. It's up the street from the comfort in, and cheaper than a proper hotel, but just as secure.
Is it possible to buy tickets the day of the match ?
Sure, but show up early just in case. If you don't love one of the top americans, don't waste your money on nosebleed seats in Ashe. Groundspasses are the way to go.