Several inexpensive, small routers are described in the thread below. Many can be powered off a "good" 5-V "USB phone charger". For a new purchase, I'd recommend at least 16 MB flash and 128 MB RAM memory so that you've got a reasonable lifespan ahead for the hardware.
Without details of your needs and budget, including 802.11ac and additional software such as VPN, it's difficult to be more specific.