For the use case where FreeSBC is used on the client side for connecting to the hosted PBX, what is the minimum hardware requirements for a small office of say 25 users?

The performance chart on the datasheet (page 3) gives you a hint as to the processor/memory requirements and scaling.  For only 25 users, a much less capable server should suffice. It is however recommended that the server be equipped with a minimum of three (3) Gigabit Ethernet ports (i.e. LAN, WAN and management). The network interface card (NIC) must use an Intel chipset. You can view a list of supported NICs here: https://docs.telcobridges.com/tbwiki/Supported_NICs