7 users is a pretty trivial load for most servers. What kind of files are you going to share from it?
If you're sharing programs (i.e., a centralized installation of Word, etc.) then you may need to be concerned about performance. The same might be true if the users are going to be doing heavy file server access for tasks such as video or photo editing. But if it's just general run-of-the mill office documents and data files, I'm don't think you'd have to be particularly concerned. I'd actually think an standard 7200RPM drive would probably be sufficient.
I hope that when you say "backup" you're talking about an external drive that's only attached to the system when you actually make backups, and which is stored separately from the system (preferably offsite). Having at least two alternating drives is even better. An online drive is not a very good backup because it's exposed to many of the same risks that the live files are.
It might help to draw a picture and label all the bandwidths/bottlenecks.
Most hard drives with small files like pictures (100KB or so, there are charts relating file size and average READ/WRITE speed. Google HDDTACH and "review") drop down to about 2MB/second average even though they can go above 100MB/second for large files.
Solid State Drives aren't needed for small setups with minimal use. I don't know about costs, but ideally you have a hot swappable setup where you have a spare hard drive laying around already screwed into the appropriate hardware and, when your Server informs you via e-mail and/or a RED LIGHT that a particular hard drive must be swapped out, you simply pull out the defective drive and insert the new one. You definitely want Hot-Swappable; you might be best served with a model where you pull out the drive tray, then switch hard drives and insert the drive tray.
You then RMA the defective drive if it's still under Warranty.
The Server will then run diagnostices on your new hard drive then copy the data over. Again, FOUR hard drives is recommended in RAID1 (they all have the same data).