Ok the two devices others have mentioned would be my gotos.
The TP-link Archer C7 should be meaty enough for your needs, 780mhz 128MB of ram, anywhere from £20-40 on ebay. Lots of people use this device so finding solutions to problems should be pretty straightforward.
The netgear R7800 if you want a fast box for what ever reason, 1.7ghz dual arm core, 512MB of ram. In the past few months they have gone for as little as £60, but you'd have be patient/get luck to get one that cheap. They seem to go from £70-100.
Was going to suggest a BT Home Hub 5a if you were handy with a serial adapter, they used to go for less than £10, but it appears all thats on ebay these days are devices people have unlocked already for ~ £20. So probably not worth it.
Then of course you could look at x86 machines (most people would call them overkill for OpenWRT will tell you to use pfsense, but I like to run a hypervisor and have openwrt in a VM, then the always on box can do more interesting things as well), but that can get expensive fast. But if you like fiddling with things can be hours of fun. You can look for thin clients with pcie slots for an extra network card, or intel NUC units with more than 1 NIC.
