You don't need a sleeper to get from Shanghai to Wuxi. I don't think you need to worry about booking in advance on this route, as long as you have a little flexibility and can do next-available. HERE is a website where you can enter those cities and come up with list of trains on this route. It's pretty up-to-date and accurate. You can take pretty much any of these trains, but if it were me I'd probably do the D's since they leave from Shanghai main station (which I presume will be more convenient for you than Shanghai South) and they originate in Shanghai. Note that trains which have a different city from Wuxi as final destination, still have Wuxi as an intermediate stop. You can click on exact train to see detailed timetable with all stops.

I'd wait to get these tickets in Shanghai, or if you don't have much flexibility, call from Chengdu to Shanghai hotel (if you know where you'll be staying by then), and have hotel get--give them 1st, 2nd, 3rd choices.