As a point of reference, https://www.gl-inet.com/products/gl-ar750s/ is a well-built, well-supported "travel router" running on a MIPS core at 775 MHz. Its performance is probably comparable to other MIPS-based units, with GL.iNet advertising 68 Mbps VPN performance with Wireguard (17 Mbps with OpenVPN).
Typically, VPN performance isn't improved significantly by multi-core devices. This suggests that a recent ARM-based unit ("mvebu", for example) or x86_64 for your VPN will be needed to reach 120 Mbps, even with Wireguard. Since one core will be swamped with VPN, you need at least a two-core device.
This, alone, is going to be challenging with a 50€ budget.
From this post and following
one of the ipq40xx devices might be able to reach 120 Mbps VPN with Wireguard.
Routing 1000/500 mbps is also a challenge, especially if you've got significant upstream usage. A solid, multi-core x86_64 with AES-NI would be highly recommended. The mvebu-based devices may be able to handle those rates, but I have no personal experience with them.