I'm thinking about going to Longquan from Nanjing.

I'm afraid there is no direct train or bus to Longquan. To Lishui, there is only one bus scheduled at the local East Bus Station and four trains running between.

While personally, I suggest you transfer at Hangzhou, which can be reached by taking the high-speed train from NJ city. Upon arrival at Hangzhou city, you can go to Hangzhou South Bus Station, where you can find direct buses to Longquan and Lishui. Therefore, why not spend one or two days at beautiful Hangzhou and then head for your destination?