Yes, you may go for a 2 TB drives because 4 TB drives are not supported on the S100 controller.
Then, do you think that RAID1 performance on S100 would be acceptable? (We use the server in our small company as Domain Controller + Network File Server).
There will be appreciable reduction in read and write performance in RAID 1 due to overhead caused by parity checks and controls for redundancy purposes.
I mean, with RAID0 file transfer is OK, but I much degradation should I expect with RAID 1: more than half speed or less?
I can not for sure provide the quantitative figure by which the performance will be degraded. While you will have good performance with RAID 0, you risk losing your data when you have a failure of a single disk because there is no redundancy of data as provided by RAID 1. Over all, the performance of software based RAID controllers such as S100 is limited by design. For better performance, I would recommend upgrading to hardware based RAID controllers like the H200.
I hope this answers you question.