Im looking at replacing a server for a customer with a NAS as their usage has changed drastically since the server went in.  They are currently running MS Server 2012 and were running a customer management system off it. They have since moved to a cloud based package for that but are still storing a ton of data on the server.  

At the moment they have 2TB of space but are adding on around 100-150GB of data per month so realistically only have about another 6 months of space left.  

Im thinking since its only for storage and the old servers aging a bit why not just replace it outright.  Im thinking a 4 drive NAS with 2x 4TB drives in it at this stage but room to expand in the future most likely with another 2x 4TB drives. The drives will be mirrored / raided / whatever the NAS supports.  

What I am needing is something that isnt going to crash regularly,  has a good backup system, ideally an incremental backup system.  That can support backing up different sets of folders on alternate days - we have 10x 2TB USB  hard drives at the moment which we will keep using for the backups - the data can be easily split as one folders photos, ones videos.  I wouldnt mind some sort of cloud based backup chucked into the works either - they are on 100/100 UFB.  

If it has a music system built in all the better but by no means a deal breaker - they have a large music collection stored on the existing server.  

Hard drive wise whats worth looking at these days?  I have traditionally used Seagate drives but got quite burnt on their whole 3TB drive fiasco a few years ago.  I see Seagate have ironwolf drives for NAS's, are these any good?  

One thing I dont want is the whole system to grind to a halt when doing things like backups - these will most likely be done overnight but you never know!  

User wise they will have upto 6 people using it at once but its really only for storage rather than them working off it constantly - mainly come into the office, transfer the photos/videos then out of the office.  Dont need any sort of remote access at this stage.