Hello,Mr.K
You can buy the ticket of train D386 from Suzhou to Beijing South Railway Station. Try to consult the railway station staff if you can purchase return ticket. Note that May 1 to May 3 is a holidy when there will be crowds of people travelling.
After getting out of Beijing Railway Station, use trolley bus no.106, or subway line 2 plus bus no.20 to get to Dongzhimen. Beijing South Railway Station is about 14km away from Dongzhimen. You can also choose to take a taxi. The taxi fare is about 35RMB.