shanghai - suzhou - does it get so busy we can't get a train ticket?

We want to visit Suzhou (with the high speed train) leaving from SH Railway Station on Sunday. Do we need to get roundtrip for fear we won't get a return seat because it may be filled up? Is it necessary? And if we do that, do we need to specify what time the return train will be? We were hoping to just buy the tickets when we're at the train station in SH and when returning from Suzhou. Thanks.