I have experience changing Japanese-market BMWs to Euro spec for use outside of Japan, and I thus speak from experience as to what needs changing when shifting a car from one market to another.
china is similar to Japan in that the nav system is a different system due to the requirement for localised language and ability to read local maps.

You will have to:

1. Remove and replace the CCC navigation computer with the Chinese version. They are very different!
2. Possibly replace rear view mirror for unit with built-in toll charge reader
3. Possibly replace telephone system for one that works in China
4. Possibly replace assist system for one that works in China
5. Reprogram vehicle by changing option codes and re-loading vehicle software

I cannot comment on vehicle legal compliance issues as I have no knowledge of how strict the Chinese market is as to vehicle compliance.
I know that when importing BMWs from Japan to Australia, New Zealand and UK, the vehicles have to go through compliance checks.
The nature and extensiveness of these legislative compliance checks vary country by country. Costs also vary.

It is quite dificult to change the new model vehicles from one market to another, as there are many many differences. The reprogramming may not work, and you will not know until you try.

Dealers will have no experience and everything will be done at your risk, and will void any waranties you may have.

If you consider shipping costs, insurance issues, compliance issues and conversion costs, I think you are honestly much better off to sell the US vehicle and purchase the local Chinese market X5.