Ah, so you want to make a network security appliance.
I would think that only one Ethernet port makes a Mac Mini a non-starter. You'd at least have to add an extra Ethernet port with some USB dongle.
I would just get one of the many small boxes and modules made today for this purpose. Most use an ARM processor.
I run OpenWRT on my wireless router. It's a fairly old one, and could use more speed - I have a pretty high-speed Internet connection (60mb/sec) and so I would get better VPN throughput (I get about 10mb/sec).
I used to use a Netscreen device between my network and the cable modem. Now the wireless router just does the job.
At some point, will probably put some dedicated little box running OpenWrt between my network and the cable modem as a security appliance. While it's popular and convenient, I do think it's kinda silly to make your wireless router do double-duty for network security because that shouldn't be tied to your wireless setup. For example, I'd like to update my wireless, but the routers that interest me the most won't run OpenWRT.
I don't think I'd use either a Mac Mini or a power-hungry old PC for this.