You can take just about any old desktop PC, having 1 or 2 GB of RAM, put in extra Ethernet cards, use a USB flash drive as the system disk, and have a system that is way overkill for almost anything you'd want to do with OpenWrt.
The downside is will use a lot of electricity, so consider it as a temporary proof of concept. Buying a network appliance x86 type system with < 10 watts power consumption will rather quickly pay for itself.
The thing to look for when buying new (besides power consumption) is does the CPU have AES-NI support, this will greatly improve the performance of a VPN server or client.