Is it right to buy the best equipments I can afford?
I believe what most are trying to illustrate is that your definition of "Best" is very different than "what is best for you to begin with."
And I think good lenses are necessary, because no matter I'm just a beginner or a master, good lenses always make photo better, right? Are you agree with me?
Normal beginner's misconception - you make the images great, not the lens. Only when your knowledge increases with years of experience will the lens start to matter. I do truly believe the 60mm f/2.8 is Nikon's best lens they make in technical terms, but it is rarely the best lens for the situation I am shooting. Lenses need to match what you are shooting. Some are more versatile for a wider range of work, while others are very specific. All of them overlap in many ways and uses. Starting out, I do not believe you will be able to tell a difference between a $2,000 lens or a $600 lens - that is until you realize that you need a different new lens. Then it comes down to if you have the money for it. All of Nikon's lenses are great when you learn to use them.
I want a better lens so I can still use it after I upgrade to fx, but not selling.
There is not issue in planning for the future and buying your most used lenses that are FX glass. I don't think you have answered the question for yourself what will your most used lenses will be. Why waste money on something that will sit on a shelf? DX ultra wide angles and most other lenses are 1/3-1/2 of their FX counterparts. The better question is; Do you want to drop a ton of money on one lens, or be able to buy multiple lenses so you can learn from many.
Of course, for camera body, a friendly and easy to control body is better than a complicated fx body. I heard that d7000 and d5100 use same CMOS? But d5100 is much cheaper than d7000, so it seems that d5100 is a better choice?
Camera's are much more than sensors. You really should visit a camera store and have the help show you the options so you can see what the differences are.
Ps: Do you think a fx body will help beginner learn faster? After all there are so many functions on professional fx body that dx body doesn't have.
Absolutely not. There is no difference in learning from a FX vs. DX camera. The difference in functionality is rather minor - and starting out, you won't even get to them before you are ready for an upgrade. I have a D800 and it has only a few additional features that I use. It makes a difference to me, but most don't ever use them. There are 100s of features I will never use, and probably a 100 more I will never know existed. Someone else may use all of those, but never will use the one's I do.
If you really want to learn, pick up a D7000 and a 35mm 1.8 and shoot non-stop so you can learn about the basics of photography and learn how to shoot. Pick up a kit zoom for convenience as well. Once you start learning, you will know what you NEED to buy, not what you think you need. Those are two very different things.
You could throw $1,000,000 at camera equipment, and it will never replace or speed up, your learning process.