Hangzhou and Shanghai are both ok. Pinghu is on the half way between Shanghai and hangzhou. If you choose Shanghai, you should get to the bus station in Zhongxing Road in the north Square of Shanghai Railway Station or Shanghai SOuth Bus Station next to Shanghai South Railway Station. Then change to buses to Pinghu, about 1.5 hour later. If you get to Hangzhou first, you should take airport bus to Wulingmen and then take a taxi to Hangzhou east Bus Station to catch buses to Pinghu. The journey is 1.5 hours.
Suggest you go to Hangzhou first then take buses at Hangzhou East Bus Station to Shaoxing and then to Yiwu and then Wenzhou. They have bus connections.