Normally I'd point towards Tips for getting cheap used x86-based firewall with full Gbit NAT (a PC Engines APU) if you are in the US as well, but with SQM at 1 GBit/s WAN speed, you're beyond that bracket (as well as beyond mvebu, mt7621/ mt7622 and ipq807x). Modern x86_64 Atom systems (N45xx/ N55xx) would probably do (up to 1 GBit/s, not beyond), but i3+ might be preferred.
However, your current needs of 100/10 MBit/s are much easier and cheaper to satisfy, so it might make sense to get something integrated (e.g. mt7622+mt7915 or ipq807x) now (up to around 100 EUR) or the aforementioned Tips for getting cheap used x86-based firewall with full Gbit NAT (a PC Engines APU) if you are in the US - and to defer the question of what might be needed for 1 GBit/s WAN speeds to the future (technology only gets better and cheaper in relation over time).