C3700. Works great but I need to extend the range. Which netgear device should I buy?

Pretty much all of netgears range extenders would work.

I would highly recommend the tri-band extenders. Reason why is a standard extender cuts throughput in half because it has to receive and re-transmit and can't do both at once. So its cuts throughput in half and increases latency. The tri-bands mititgate that somewhat by reserving a chip just for router-extender communcations.