Pro is tested on 250 clients, but Go can connect concurrently 100. (These are connections, not users)
Web Direct needs many resource, but your case seems not so much.
Recommending the right hardware is impossible without a lot more detail. User19752 pointed out the minimum and recommended specs but I would say that even the recommended ones are on the low side.
Especially around the processing power side. How well FMS uses the available processing power depends a lot on how the solution is designed and how intensively you are making use of PSoS calls and server-side schedules. Once you go past 50 concurrent users you will need a machine with plenty of cores.
Disk space and size are relatively easy: go for SSD and dimension them so that you'll have plenty of space for your live files and all the backups you want to keep.
Of course if you want to add WebDirect to the mix then it becomes a completely different ballgame.
If you are not very familiar with FMS performance monitoring and you do not have a good current baseline of how your solution performs then you may want to stick with Windows servers and pick them so that you can easily add processors and other hardware as you start seeing the actual performance numbers come in.