Check out https://www.ebay.co.uk/itm/325210818105 , you can get one for less, if you already have a spare NIC lying around somewhere.
The CPU should be able to deliver somewhere around 400-500/mbit, perhaps more, if the NIC it comes with, have offloading.
Or https://www.ebay.co.uk/itm/284401530787, using an Atom CPU, and intel 211s.
https://www.ebay.co.uk/itm/203894705853, if you think you've got a fitting power adapter.
These should easily route 1gbit, read about it here.
The HP T610 is another candidate, add a USB3 NIC for the thin version, or a pcie card for the Plus model.
then an AP as @mercygroundabyss recommended.