Depending on how the software handles the data writes, you won't want 10K RPM drives, you'll want an SSD. Far more responsive and sustained reads are very fast; the only downside in a server set-up for an SSD is if the workload is dealing with huge amounts of small file writes (though standard HDDs have trouble with this too) or that it writes and re-writes a huge amount of data over short periods of time, in which case you may kill the SSD rather quickly.
Since this is accounting software I would assume (could be wrong) that this is all just basic data entry and report generation, as the reports are likely just stored in RAM as the system is utilized and then dumped later, I see this as an ideal solution for an SSD.
In fact I think you may want to experiment with grabbing an SSD, and cloning the drive that's in the current machine. As the SSD can very easily be re-purposed into whatever server you end up buying/building, this would be a great way to see if maybe the generic 5400 RPM low capacity drive is just struggling to deal with the simultaneous read requests over different sectors on the drives.
The following is going to sound like a bit of a repeat of Bonzai's post
here but this is my own thought process.
Based off the information I've read about the current machine, the number of current users hitting the system, and the issues you're having here's the breakdown I ran in my head:
1. CPU? Generic i5, but even a first gen i5 desktop CPU should be able to generate accounting reports fairly easily. Unless these reports are HUGE or involve thousands of entries (highly recommend you ask or investigate this) then likely not the issue.
2. RAM? 8GB is way more than enough for the guy using the system, and with only 3 concurrent users this again points to the size of the reports being generated or how long the software lets the data sit idly in the RAM before it dumps it. One thing to consider here though aside from the size of the reports it's generating is that it's likely storing this data, the software itself, AND whatever the current user is working on. 8GB for one person in an office environment is usually overkill but this could quickly lead to an issue in your configuration as then it relegates the overflow to a paging file on the HDD which ALWAYS makes operations dog slow.
3. HDD? Most likely culprit in my mind, when you're using a mechanical drive, have a current user, software that needs to constantly write/read to different sectors WHILE having multiple other people request data, a single mechanical drive is going to suffer a lot doing this. This is only made worse by the possible paging file overflow from the RAM possible issues.
4. Network? Networking can be a troublesome beast to deal with, as all equipment is rated in its max theoretical speed but realistically never hitting it. This being the issue is COMPLETELY reliant on the size of the data we're dealing with.
If these reports are ~10 MB (that's a lot of basic numbers/data when you really look at it) then over a fast ethernet connection the theoretical time would be <1 second. But realistically this'll take a couple of seconds, and with dated networking equipment anywhere in the chain this could take a minute or two, not something I'd say worth complaining about unless they need this information constantly being pulled.
If these reports are ~100MB or larger (that's HUGE for accounting reports/files imo) then the network could easily be an issue, as network connection speed for internal file transfers has a large number of variables to account for. Multiple people asking with dated networking equipment are going to be very annoyed if they need this data often as they'll spend most of their day waiting.
Sort of typed all that before looking into the most recent version of Sage 50, I only see these possibilities:
1. The guy using the station is running too many things concurrently and the memory is all used up, causing the accounting program to use paging files; not super likely but possible!
2. Most likely in my opinion is that the drive in this machine is rather slow, and trying to service multiple people numerous data fetch requests, while the guy is using it/accessing files on it, while the software does whatever it does in the background.
Solution to 1 is just isolating the machine, his machine would be overkill for the software if left alone for only Sage 50.
Solution to 2 is an SSD honestly, the near instant seek times for this environment is going to be extremely useful, and its read speed throughput is going to be a dramatic increase over your current drive.
tl;dr, grab an SSD and clone the guy's drive (though I'd recommend overprovisioning an buffer so he doesn't write amplify it to death, maybe format it with only 85 or 90% of the usable space) and see if that doesn't remedy the problem.