I will be arriving in Hangzhou Xiaoshan International Airport at night around 10+pm in October.

Do you refer to Sofitel Westlake Hotel? If yes, you may take shuttle bus Pinghai Road Line to the terminus: Grand Metropark Hotel. The trip takes about 70 minutes, and the fare is CNY 20. Upon arrival, take a taxi to the hotel for CNY 11.

How much will it cost if I will to take taxi from the airport to Sofitel Westlake Hotel? Thank you.

The fare may be CNY 70 to 80. ;)

To travel from HZ to Suzhou, may I know :
1) which train station in HZ (nearest to Sofitel Westlake Hotel) should I go to take the bullet train to Suzhou?
2) can I purchase train ticket at the counter on day of travel to Suzhou?

Thanks.

You should walk east for about 700 yards to Ding'an Road Metro Station, and take metro line 1 to East Railway Station. Then, take a bullet train to Suzhou (about 1.5 to 2 hours).. The tickets might be sold out on the day of travel. Thus, you are advised to book it in advance through bookingtravelchinaguide.com.

I would like to visit Humble Administrator's Garden and GuanQian Street in Suzhou. May I know which train station should I get off in Suzhou to go to these two places?

You should get off at Suzhou Railway Station. Take bus line Haoxing 7 to the garden. After visiting, take bus line Haoxing 5 to Guanqian Street. :)

For the railway station at Suzhou, which specific railway station should I alight:
1) Suzhou (Anhui) or
2) Suzhou (Anhui) East or
3) Suzhou (Jiangsu) or
4) Suzhou (Jiangsu) Industrial Park or
5) Suzhou (Jiangsu) New District or

You should alight Suzhou (Jiangsu) Railway Station. :)