Yes sorry I didn't make it clear, this is an RDS server so the .exe and DBFs will be on the same machine. So it sounds like the main consideration to make the application work as quickly as possible is the disk.
Typically in the past we would have had 4 15k rpm HDD's running in a Raid 10 configuration. We are thinking maybe SSD or NVMe will make a massive difference but not sure how much of a difference taking into account the cost.
VFP is IOPS intensive, so anything you can do to increase that number will directly benefit VFP applications. An array of 4 SSDs equals to an array of 150 15K SAS disks in terms of IOPS. If SSD is an option when considering that server grade SSDs are more expensive, I'd definitely go with SSD. I don't think NVMe would make a big difference here over 6 Gb/s SSDs. Depending on how many different channels you have to access SSDs, they already deliver 15-60 MB/s per user minimum. Plus a TS with local DBF files can actually cache data that is most used unlike TS that access a NAS or SAN.
The other issue is memory. Without reconfiguring Window a VFP application can make use of up to 2 GB of memory. For 30 users working mainly with this application you probably peak out at 96 GB RAM. It's a little more if you run browsers and Office in addition to the VFP application. On the other side of the spectrum I wouldn't run on anything less than 32 GB these days.