Starting off simple: if you just want something that will NAT 2Gb/s and pass whatever traffic to your server, and you have little to no plans to do anything more on the box, a simple Cisco RV340 would suffice. It's GUI-based and well-supported, although completely locked to Cisco's in-house distro and feature set. A Ubiquiti EdgeRouter 4 would allow for more functionality, as it exposes basically a full Vyatta stack to you, and you can install Debian packages to customize it (although with only a 1Ghz MIPS CPU, it's somewhat limited as to what it can run). You'd also need a switch with that router, as it's just a pure router (the EdgeRouter 12, with a built-in switch chip, will only run v. 2.x of the EdgeOS firmware, which still isn't stable enough yet IMHO). Mikrotik RouterOS would be less extensible, but a more fully-featured platform for pure routing and IP feature set (hardware: RB3011/4011/1100AHx4/CCR).
For max performance and flexibility, but still low-ish power draw, I'd go embedded x86 with Intel NICs, such as a Qotom Q555G6 (
bare-bones for ~$330 -- you supply the RAM and mSATA SSD, which I like better because the flash they tend to ship with is mediocre at best). Power draw should be well under 20 watts average. The i5-7200U will get you well over a gigabit's worth of
software/CPU-driven throughput (ie. run anything you want to, offloadable or non-offloadable, and you don't have to care), versus lower-wattage but less powerful architectures (MIPS/ARM -- ie. EdgeRouters or Mikrotik) that would rely on hardware-offloading or else likely wouldn't have the CPU to run many items at gig speed (QoS, OpenVPN, Surricata IDS/IPS, reporting, etc.). Plus, on generic x86 hardware, you're free to try a myriad of distos until you find one you like: pfSense/OPNsense, OpenWRT, IPFire, Endian, VyOS, Untangle... the list is literally endless. If you're less cavalier about custom-constructing your own solution and/or want a bit more friendly user experience created for you, you could opt for a pre-built appliance, such as
Firewalla Gold, a
Netgate pfsense SG-series or an
Untangle z-Series.
So it really comes down to how simple or flexible you want this box to be. Best blend of simplicity, reliability and price would probably be a Cisco RV340. Beyond that, I would probably lean towards a low-power x86 box, per my guidance above.