As far as the storage portion of your post goes, Heroes of the Storm, games that are similar to it, and open world type games in general see load time benefits from SSD tech, especially when loading up new areas of these expansive maps. SSD is king for performance, but still quite expensive for the amount of storage space you're getting, so it just depends on where your values land in terms of cost vs performance.
One solution which would be a good fit, if you feel that the cost of SSD is prohibitive, is an SSHD or Solid-State Hybrid Drive (current Seagate model:
FireCuda). These drives have a small SSD cache, the drive looks at what data you're accessing most frequently and places them on this portion, then also has a larger spinning disk storage capacity for the rest of your data so that you still get solid bang for your buck on storage space.
If you'd like to see how these drives stack up comparatively, here are a couple of charts you may find useful:
The first one compares startup times across several popular games across a traditional spinning 7200 RPM HDD, our SSHD, and an M.2 SSD (128GB). The white is for SSD, the orange for our SSHD, and the gray for the 7200 spinning HDD.
Startup Times
The next one compares the first 3 days of gaming storage utilization on several popular titles, and SYSmark ratings from various drive types and combinations. First of the grays is 7200 RPM 1TB spinning HDD, second (lightest gray) is our SSHD, third (darkest gray) is an SSD + 7200 RPM HDD combo, purple is SSD + our SSHD combo, and lastly blue is SSD.
First 3 Days Gaming Storage Utilization
Good luck wiith your new build, regardless of which drive you determine is right for your needs in the end!