How about a used Dell Optiplex 9020 SFF ?
https://www.ebay.com.au/itm/Dell-OptiPlex-9020-SFF-Core-i7-4770-3-4GHz-8GB-Ram-128GB-SSD-Win-10-P/253373075009?hash=item3afe364e41:g:1-UAAOSwlAZaT41m
4 core i7 (up fo 3.9 GHZ) with , 8GB RAM + 120GB SSD, plus a Win10 license for 350$AU
Add a chinese knock-off i350-T4 network card for 60$AU.
The i5 version is available for only 270$...
I run the i7 version with ESXi and 16GB RAM. It handles a 50/20 NBN connection withoit breaking a sweat... I have tried OVPN and could achieve 45Mbit/sec during the day.
At night times my provider or NBN starts dropping UDP packets and VPN throughput becomes unusable at 1.5 MBit/sec. Back to 45 again after 11pm.