C+P+F+A=$
There's a formula that I like to use when architecting data storage (such as Veeam repositories): "C+P+F+A=$", where...
C: Capacity - "How much data storage will you need? What are your data compression and data deduplication expectations, if any?"
P: Performance - "What are your storage performance metrics (how fast would you like it to be)?" What type of workload are you architecting for (for example: Small block random reads/random writes, large block sequential writes). Are you more concernd about IOPS or are you more concerned about throughput(MB/sec)?
Note: Backup and archive workloads are most often sequential, but can, at times, be random.
F: Features - "What do you plan to do with your data (snapshots, replication, etc...)? Do you want your data to be immutable? What about tiering your data to less expensive disk over time? What about data archival?"
A: Availability - "How important is the availability and continuity of the data to you?"
$: Investment - "How much will you need to invest in data storage?"