the $48 ER-X is much faster than 99% of peoples' residential last mile broadband connections, it's good for up to about 750 Mbps of NAT and default route outbound to a gateway.
I have no problems with a gigabit symmetrical line on ERLite-3. UniFi Security Gateway is the same hardware but in a nicer interface that works with UniFi APs & Switches if you want to go that route but you have to also host a controller. You can also upgrade to a ER-4 for a much faster CPU but I don't think you need to.