All modern SA have a real time FFT mode. The span depends on the BW of the real time module. They also demodulate, do CCDF, look at pulses and have a wide selection of measurements. They go down to 9kHz, but you are right, not all the way do DC. There are even some that go down to 5Hz. I agree, to do real time processing in the back, you can use the Oscilloscope as a front end, but then FFT is done on the PC, not the scope...I also use it this way for modulation analysis.