I'm looking for a NAS for business/ office use with the following requirements:-
- Raid 5
- 4 slots (I need about 1-2TB)
- Good file writing speed
- Active Directory support (users are on AD therefore the file server has to be on AD as well)
- Low maintenance

May I know if there are NAS which comes with it's own OS? If possible I would like to save on buying an OS for the NAS.

I'm currently interested in the Synology DS411+ but are there any other NAS in the same class which you would recommend? The DS411+ is about 2.5k but if possible I'm looking for one below RM2k.