What will be the weather for Mid-March? It's fine to go up to HuaShan then?

It will be cold on the mountain. The temperature may be around 10C. It can down very low at night and on the top. You'd better take down jacket. Actually, May may be the best time to climb the mountain.

Thank you for your advice. I have a business trip to Guangzhou on 3-7 March. Therefore, I plan to go Xi'an first & at the same time to climb Mt Huashan. What will be the best time for me to leave the city for me to able to climb the mountain & be back to the City by 6-7pm?

How long will be the journey from the city to the mountain? By bus or by train?

It is suggested to leave the city in the afternoon and to climb the mountain at night. Then go back to the city next noon by taking bulle trains.
It is suggested to take the bullet trains to finish your journey, which will take you about 40 minutes.