Had to move up my trip.
Ended up going to Shanghai Zhan to buy my Wuxi tickets on the same morning, to save the hassle of buying tickets the night before and paying a service charge.
My first choice was sold out but I got the next available train 15 minutes later. Unlike the first class seat trains, my Dxxxx train to Wuxi was a sleeper train and only sold second class tickets for 39 kuai. I couldn't find my seat initially, so I just found myself an empty compartment which I had to myself for the entire 60 minute journey. It was an early morning departure and I just did some work on the laptop. I was pretty impressed by the quality of the train. There was central AC as well as LCD TVs in the compartment. A big upgrade from when I used to take hard sleepers 20 years ago.
At Shanghai Zhan, a 40 kuai taxi ride from my hotel in Pudong, there are self service ticketing stations which allow you to buy return tickets. Just make sure you bring crisp notes. The queues are hit and miss but moved fairly quickly at 8am in the morning.
The return Dxxxx train took 70 minutes as it stopped in Suzhou and Kunshan. First class seat was 47 kuai and seating is 2-2. I slept the entire way back.
I took the metro back from Shanghai Zhan to Peoples Park on the return and it was the least pleasant of my 20-odd metro rides over two days.