Need a device to calculate the FFT more than 100 times per second (my scope is maxed out at 10hz)
my input waveforms range from 100hz to 10,000hz.


Seriously, that means (by Nyquist) that a sampling rate of 20 kHz is sufficient – you probably want to oversample a bit, but everything above 40 kHz is luxury.

In other words, this is best soundcard-territory. 

Get a cheap (&lt;10$) USB sound card, remove the coupling capacitors from the microphone input, and use some kind of of adjustable opamp-based amplifier to get your input signal into the Soundcard's range. Done.


  It would be difficult to use the soundcard for live FFT visualization (especially on something like a Raspberry PI)


You're making wrong assumptions here!

Processing this amount of data is definitely not a problem on anything faster than the cheapest pocket calculators. A raspberry Pi, a laptop, a PC or a smartphone will definitely do. Really, the effort of doing an FFT on this amount of samples is thoroughly ridiculously small. Decoding an MP3 is far more tasking for any computer (fun fact: MP3 requires an operation pretty similar to the FFT).

Don't worry about Python slowing you down: No Python library I know does the FFT in python itself, but rely on FFTpack or FFTw (numpy, for example). You can do such short FFTs (i.e. "less than a hundred thousand bins") at sampling rates of multiple Megasamples per second on a normal PC, so don't worry about the workload posed by the FFT. Seriously, 100+ transforms of length 1024 per second? More like 1 Million transforms per second, in my experience. I can't stop wondering why people even think that's hard, but watch full-screen MPEG4-encoded videos at the same time on all of their devices. 

To demonstrate how easy this is: I wrote an answer a while back that demonstrates how to visualize audio data. If you don't do the whole loopback dance, but just select your soundcard as source, you're done and have your FFT'ing visualization:



I also wrote a minimal firmware a while back (can't find it, sorry) that used a 6$ Tiva-C ARM evalboard to convert and send samples to a PC at up to 200kS/s over USB as bulk transfers. You'd have to write your own userland "driver" software to talk to such a device, but it'd work, too. But there's not much winnage in doing so compared to buying a sound card, unless you'll need to use such a microcontroller anyway, e.g. to control something else.