You should go to Shanghai or Hangzhou first from Singapore and there are direct buses and trains to Huangshan from these two cities. You can also take flight from Shanghai to Huangshan. Travel by bus is recommended. In December, Tiandu Peak and West Sea Vally may be closed. No problem to visit other places in the Mountain. There are cable cars which can make your trip easier. I think two days in Huangshan is enough if you catch the earliest bus from Hangzhou. You can visit Thousand Island Lake nearby Hangzhou, West Lake in Hangzhou and Water Towns around.