What is recommended time between international arrival at Hongqiao T1 to fast train near T2?

Roel, generally 2h should be enough. It is suggested you book the train tickets in advance and you only need to collect them at the ticket office which can save some time. You can book the train tickets via TravelChinaGuide by emailing to this address: bookingtravelchinaguide.com. I did it once. It is quite convenient.
Oh, I remember the ticket check stops 3min before the departure time. Make sure you board the train on time.