re: Need "C"Wire (5th wire) for Wifi thermostat.

Yes that should be 24V. One of the browns should be connected to the wires that go outside to the condenser. Probably with a wire nut, you can just add the blue one to that, but there is a pretty good chance that one of those three brown wires is not connected to anything. I think the fan relay is on the board, so one brown is probably the common for the burner, one brown should go outside to the condenser and the other brown one should be available for the thermostat.